第一节　　叶的生理功能与外部形态

一、叶的生理功能

  叶的功能主要是光合作用和蒸腾作用，有些植物的叶片还有储藏和繁殖的功能。

(一) 光合作用(photosynthesis)

植物的叶有规律地生于枝上,担负着植物生活中最重要的生理功能一一光合作用。光合作用就是植物的绿色组织通过叶绿体内的有关色素和酶类，利用太阳光能，把二氧化碳和水合成有机物( 主要葡萄糖)，并将光能转化为化学能储藏起来，同时释放氧气的过程。光合作用为植物生长、发育提供所需的葡萄糖，也是植物进一步合成淀粉、脂类、蛋白质、核酸、纤维等有机物质的重要原料。

  (二)蒸腾作用(transpiration )

蒸腾作用是植物体内水分以气体形式从植物体表面散失到大气中的过程。植物一生所吸收的水分中，大约99% 通过燕腾作用散失到大气中，只有1% 成为植物体组成成分。蒸腾作用产生的蒸腾拉力是植物吸收和运输水分的主要动力，特别是高大植物，如果没有蒸腾作用，较高的部分很难得到水分; 蒸腾作用引起的上升液流，有助于根吸收的矿质元素以及根中合成的有机物转运到植物体的其他部分。另外，蒸腾作用还能够降低叶片温度，避免叶温过高，对叶片造成灼伤。

叶片还有吸收的能力，如向叶面喷洒一定浓度的肥料( 根外施肥) 和喷洒农药，均可被叶表面吸收。
　　有些植物的叶还能进行繁殖，在叶片边缘的叶脉处可以形成不定根和不定芽。当它们自母体叶片上脱离后，即可独立 形成新的植株，可用来繁殖某些植物，如柑橘属、秋海棠属植物。

二、叶的外部形态

 (一)叶的组成
  典型的叶由叶片( leaf blade)叶柄( petiole) 和托叶(stipule)三部分组成( 图4-1)。叶片是最重要的部分，一般为薄的扁平体，这一特征与它的主要生理功能一一光合作用相适应。叶片内分布的叶脉具有支持叶片伸展和输导水分与营养物质的功能; 叶柄位于叶片基部，并与茎相连，具有支持叶片的功能，使叶片伸展在一定的空间位置以接受较多阳光，同时联系叶片与苍之间水分与营养物质的输导; 托叶位于叶柄和茎的相连接处，具有保护幼叶的作用，通常早落。有些植物托叶变为鞘状，如蓼科植物; 有些植物托叶与叶柄合生，如蔷薇属植物; 有些植物的托叶较大，如鹅掌楸; 有的植物托叶生活较长时间，与叶片近乎等大，如豌豆; 也有些植物的托叶变为刺状或卷须，如刺槐。

lateral:侧面的

adnate:贴生的

interpetiolar:同叶柄间的

foliaceous:叶状的

ochreate:具托叶鞘的

tendrillar:卷须状的

ventral:腹部的

下面是几种托叶的实物：

具叶片、叶柄和托叶三部分的叶，称为完全叶(complete leaf)，如榆、梨，棉花、桃、豌豆等( 图4-1); 而缺少其中任何一部分或两部分的叶称为不完全叶(incomplete leaf),如甘薯、油菜、向日葵、丁香等的叶缺少托叶，烟草、莴苣、荠菜等的叶缺少叶柄和托叶，还有些植物的叶甚至没有叶片，只有一扇化的叶柄着生在茎上，称为叶状柄(phyllode),如台湾相思树等。

(二)禾本科植物叶的特点

水稻、小麦等不本科植物叶的组成与上述不同,由叶片、叶鞘(leaf sheath)、叶枕、叶舌(ligulate)、叶耳(auricle)组成。叶片条形，具平行脉。叶基部呈鞘状，叶鞘一侧开裂，包围着茎杆，有保护茎的居间生长、加强茎的支持作用及保护叶腋内幼芽的功能。叶鞘与叶片连接处的外侧称为叶枕(pulvinus)( 又称叶颈、叶环)，它是- -个与叶片颜色不同的环，具有弹性及延伸性，可以调节叶片的位置。在叶鞘与叶片连接处的内侧，有些禾本科植物有一向上突起的膜状结构，称为叶舌; 叶舌能使叶片向外弯曲，使叶片接受更多的阳光，同时可以防止水分、病原菌反害虫进人叶朝内。有些相植物，在叶舌的两旁，有一对从叶片基部边缘伸出来的突出物，称为叶耳。叶舌、叶耳的有无、形态、大小及色泽常为禾本科植物分类的依据。如小麦叶耳明显，稗草则不具叶耳( 图4-2)。

(三)单叶和复叶

1.单叶( simple leaf)

指一个叶柄上只生1个叶片，如棉、油菜、桃等的叶。

2.复叶( compound leaf)
  指一个叶柄上着生2 个以上分离的叶片，如大豆、蚕豆、紫云英、七叶树等
的叶。复叶的叶柄称总叶柄(common petiole), 其延伸的部分称叶轴( rachis)。其上着生的叶片称小叶( leaflet )，小叶的柄称为小叶柄( petiolule),小叶的托叶称小托叶(stipel )。

  (四)叶的镶嵌
  叶在茎上的排列，不论是哪一种叶序，相邻两节的叶，总是不相重叠而成镶嵌状态,这种同一枝上的叶，以镶嵌状态的排列方式而互不重叠的现象，称为叶镶嵌( 图4-3)。

(五)异形叶性
　　一般情况下，一种植物具有一定形状的叶，但有些植物，却在一个植株上有不同形状的叶。这种同一植株上具有不同叶形的现象，称为异形叶性。异形叶性的发生，有的植物因枝的老幼不同而叶形各异，例如，圆柏幼树叶全为刺形，随树龄增长，刺形叶逐渐被鳞形叶代替。白菜、油菜，基部的叶较大，有显著的带状叶柄，而上部的叶较小，无柄，抱茎而生。有的植物由于受外界环境的影响，形成异形叶性，例如，慈姑有三种不同形状的叫:气生叫箭形、漂浮叫椭圆形、沉水叶带状;又如，水毛茛气生叶扁平广阔、沉水叶细裂成丝状，这些都是生态的异形叶性( 图4-4)。

资料一：植物叶的形态

资料二：

二、英文诗一首

The Shapes of Leaves

Ginkgo, cottonwood, pin oak, sweet gum, tulip tree:

our emotions resemble leaves and alive

to their shapes we are nourished.

Have you felt the expanse and contours of grief

along the edges of a big Norway maple?

Have you winced at the orange flare

searing the curves of a curling dogwood?

I have seen from the air logged islands,

each with a network of branching gravel roads,

and felt a moment of pure anger, aspen gold.

I have seen sandhill cranes moving in an open field,

a single white whooping crane in the flock.

And I have traveled along the contours

of leaves that have no name. Here

where the air is wet and the light is cool,

I feel what others are thinking and do not speak,

I know pleasure in the veins of a sugar maple,

I am living at the edge of a new leaf.

The author

Born in New York City in 1950, Arthur Sze is the author of nine books of poetry, including Compass Rose (Copper Canyon Press, 2014). He served as a Chancellor of the Academy of American Poe

三、叶的形态

Glossary of leaf morphology

Leaves of most plants include a flat structure called the blade or lamina, but not all leaves are flat, some are cylindrical. Leaves may be simple, with a single leaf blade, or compound, with several leaflets. In flowering plants, as well as the blade of the leaf, there may be a petiole and stipules; compound leaves may have a rachis supporting the leaflets. Leaf structure is described by several terms that include:

Image	Term	Latin	Description
	bifoliolate		Having two leaflets[1]
	geminate
	jugate
	bigeminate		Having two leaflets, each leaflet being bifoliolate
	bipinnate	bipinnatus	The leaflets are themselves pinnately-compound; twice pinnate
	biternate		With three components, each with three leaflets
	imparipinnate		With an odd number of leaflets, pinnate with a terminal leaflet (the opposite of paripinnate)
	odd-pinnate
	paripinnate		Pinnate with an even number of leaflets, lacking a terminal leaflet (the opposite of imparipinnate)
	even-pinnate
	palmately compound	palmatus	Consisting of leaflets all radiating from one point
	pinnately compound	pinnatus	Having two rows of leaflets on opposite sides of a central axis, see imparipinnate and paripinnate
	simple		Leaf blade in one continuous section, without leaflets (not compound)
	ternate	ternatus	With three leaflets
	trifoliate	trifoliatus
	trifoliolate	trifoliolatus
	tripinnate	tripinnatus	Pinnately compound in which each leaflet is itself bipinnate

Leaf and leaflet shapes[edit]

Being one of the more visible features, leaf shape is commonly used for plant identification. Similar terms are used for other plant parts, such as petals, tepals, and bracts.

Oddly pinnate, pinnatifid leaves (Apium graveolens, celery)

Partial chlorosis revealing palmate venation in simple leaves of Hibiscus mutabilis

Image	Term	Latin	Refers principally to	Description
	acicular针状的	acicularis	entire leaf	Slender and pointed, needle-like.
	acuminate尖形的	acuminatus	leaf tip	Tapering to a long point in a concave manner.
	acute尖的		leaf tip or base	Pointed, having a short sharp apex angled less than 90°.
	apiculate	apiculatus	leaf tip	Tapering and ending in a short, slender point.
	aristate	aristatus	leaf tip	Ending in a stiff, bristle-like point.
	attenuate	attenuatus	leaf base	Having leaf tissue taper down the petiole to a narrow base, always having some leaf material on each side of the petiole.
	auriculate	auriculatus	leaf base	Having ear-shaped appendages reaching beyond the attachment to the petiole or to the stem (in case of a seated leaf).
	asymmetrical		entire leaf	With the blade shape different on each side of the midrib.
	caudate	caudatus	leaf tip	Tailed at the apex.
	cordate, cordiform	cordatus	entire leaf	Heart-shaped, with the petiole or stem attached to the notch.
	cuneate	cuneatus	leaf base	Triangular, wedge-shaped, stem attaches to point.
	cuspidate	cuspidatus	leaf tip	With a sharp, elongated, rigid tip; tipped with a cusp.
	deltoid, deltate	deltoideus	entire leaf	Shaped like Greek letter Delta, triangular, stem attaches to side.
	digitate	digitatus	entire leaf	With finger-like lobes, similar to palmate.[2]
	elliptic	ellipticus	entire leaf	Oval, with a short or no point.
	ensiform	ensiformis	entire leaf	Shaped like a sword, long and narrow with a sharp pointed tip.
	emarginate	emarginatus	leaf tip	Slightly indented at the tip.
	falcate	falcatus	entire leaf	Sickle-shaped.
	fenestrate	fenestratus	surface features	Large openings through the leaf, see perforate. Sometimes use to describes leaf epidermal windows.
	filiform	filiformis	entire leaf	Thread- or filament-shaped.
	flabellate	flabellatus	entire leaf	Semi-circular, or fan-like.
	hastate	hastatus	entire leaf	Spear-shaped: Pointed, with barbs, shaped like a spear point, with flaring pointed lobes at the base.
	laciniate	lacinatus	entire leaf	Very deeply lobed, the lobes being very drawn out, often making the leaf look somewhat like a branch or a pitchfork.
	lanceolate	lanceolatus	entire leaf	Long, wider in the middle, shaped like a lance tip.
	laminar		3-d shape	Flat (like most leaves)
	linear	linearis	entire leaf	Long and very narrow like a blade of grass.
	lobed	lobatus	entire leaf	Being divided by clefts, may be pinnately lobed or palmately lobed.
	lorate	loratus	entire leaf	Having the form of a thong or strap.
	lyrate	lyratus	entire leaf	Shaped like a lyre, pinnately lobed leaf with an enlarged terminal lobe and smaller lateral lobes. See also: List of lyrate plants
	mucronate	mucronatus	leaf tip	Ending abruptly in a small sharp point as a continuation of the midrib.[3]
	multifid	multi + findere	entire leaf	Cleft into many parts or lobes.
	obcordate	obcordatus	entire leaf	Heart-shaped, stem attaches at the tapering end.
	oblanceolate	oblanceolatus	entire leaf	Much longer than wide and with the widest portion near the tip, reversed lanceolate.
	oblique		leaf base	Asymmetrical leaf base, with one side lower than the other
	oblong	oblongus	entire leaf	Having an elongated form with slightly parallel sides, roughly rectangular.
	obovate	obovatus	entire leaf	Teardrop-shaped, stem attaches to the tapering end; reversed ovate.
	obtrullate		entire leaf	Reversed trullate, the longer sides meet at the base rather than the apex.
	obtuse	obtusus	tip	Blunt, forming an angle > 90°.
	orbicular	orbicularis	entire leaf	Circular.
	ovate	ovatus	entire leaf	Oval, egg-shaped, with a tapering point and the widest portion near the petiole.
	palmate	palmatus	entire leaf	Palm-shaped, i.e., with lobes or leaflets stemming from the leaf base.[4]
	palmately lobed	palmatus	entire leaf	Lobes spread radially from a point. [5]
	palmatifid	palma + findere	entire leaf	Palm-shaped, having lobes with incisions that extend less than half-way toward the petiole.
	palmatipartite	palma + partiri	entire leaf	Having lobes with incisions that extend over half-way toward the petiole.
	palmatisect	palma + secare	entire leaf	Having lobes with incisions that extend almost up, but not quite to the petiole.
	pandurate	panduratus	entire leaf	Fiddle-shaped; obovate with a constriction near the middle.
	pedate	pedatus	entire leaf	Palmate, with cleft lobes.[6]
	peltate	peltatus	stem attachment	A round leaf where the petiole attaches near the center. An example would be a lotus leaf.
	perfoliate	perfoliatus	stem attachment	With the leaf blade surrounding the stem such that the stem appears to pass through the leaf.
	perforate	perforatus	leaf surface features	Many holes, or perforations on leaf surface. Compare with fenestrate.
	pinnately lobed	pinna + lobus	entire leaf	Having lobes pinnately arranged on the central axis.
	pinnatifid	pinna + findere	entire leaf	Having lobes with incisions that extend less than half-way toward the midrib.
	pinnatipartite	pinnatus + partiri	entire leaf	Having lobes with incisions that extend more than half-way toward the midrib.
	pinnatisect	pinnatus + sectus	entire leaf	Having lobes with incisions that extend almost, or up to midrib.
	plicate	plicatus	3-d shape	Folded into pleats, usually lengthwise, serving the function of stiffening a large leaf.
	reniform	reniformis	entire leaf	Shaped like a kidney: an oval with an inward curve on one side.
	retuse		leaf tip	With a shallow notch in a round apex.
	rhomboid, rhombic	rhomboidalis	entire leaf	Diamond-shaped.
	rounded	rotundifolius	leaf tip or base	Circular, no distinct point.
	semiterete		3-d shape	Rounded on one side, but flat on the other.
	sinuate	sinuatus	3-d shape	Circularly-lobed kind of leaves
	sagittate	sagittatus	entire leaf	Arrowhead-shaped with the lower lobes folded, or curled downward
	spatulate	spathulatus	entire leaf	Spoon-shaped; having a broad flat end which tapers to the base
	spear-shaped	hastatus	entire leaf	see hastate.
	subobtuse	subobtusus	leaf tip or base	Somewhat blunted, neither blunt nor sharp
	subulate	subulatus	leaf tip	Awl-shaped with a tapering point
	terete		3-d shape	Circular in cross-section; more or less cylindrical without grooves or ridges.
	trullate		entire leaf	Shaped like a bricklayer's trowel
	truncate	truncatus	leaf tip or base	With a squared-off end
	undulate	undulatus	3-d shape	Wave-like
	unifoliate	unifoliatus	compound leaves	With a single leaflet. It is distinct from a simple leaf by the presence of two abcission layers and often by petiolules and stipels

Edge[edit]

Leaf margins (edges) are frequently used in visual plant identification because they are usually consistent within a species or group of species, and are an easy characteristic to observe. Edge and margin are interchangeable in the sense that they both refer to the outside perimeter of a leaf.

Image	Term	Latin adjective	Description
	entire	Forma integra	Even; with a smooth margin; without toothing
	ciliate	ciliatus	Fringed with hairs
	crenate	crenatus	Wavy-toothed; dentate with rounded teeth
	crenulate	crenulate	Finely crenate
	crisped	crispus	curly
	dentate	dentatus	Toothed. May be coarsely dentate, having large teeth, or glandular dentate, having teeth which bear glands
	denticulate	denticulatus	Finely toothed
	doubly serrate	duplicato-dentatus	Each tooth bearing smaller teeth
	serrate	serratus	Saw-toothed; with asymmetrical teeth pointing forward
	serrulate	serrulatus	Finely serrate
	sinuate	sinuosus	With deep, wave-like indentations; coarsely crenate
	lobate	lobatus	Indented, with the indentations not reaching the center
	lobulate	lobulatus	With small lobes
	undulate	undulatus	With a wavy edge, shallower than sinuate
	spiny or pungent	spiculatus	With stiff, sharp points such as thistles

Leaf folding[edit]

Leaves may also be folded or rolled in various ways. If the leaves are initially folded in the bud, but later unrolls it is called vernation, ptyxis is the folding of an individual leaf in a bud.

Image	Term	Latin	Description
	carinate or keeled	carinatus	with a longitudinal ridge
	conduplicate		folded upwards, with the surfaces close to parallel
	cucullate		forming a hood, margins and tip curved downward
	involute		rolled upwards (towards the adaxial surface)
	plicate	plicatus	with parallel folds
	reduplicate		folded downwards, with the surfaces close to parallel
	revolute		rolled downwards (towards the abaxial surface)
	supervolute		opposing left and right halves of lamina folded along longitudinal axis, with one half rolled completely within the other