第四节 双子叶植物茎的次生生长及次生结构

　　茎的次(侧)生分生组织细胞的分裂、生长和分化使茎加粗的过程叫次生生长，次生生长所形成的次生组织组成次生结构。茎的次(侧)生分生组织，包括维管形成层和木栓形成层。，多年生双子叶木本植物，不断地增粗和长高，必然需要更多的水分和营养，同时，也需要更大的机械支持力，因此必须相应地增粗即增加茎的次生结构。

一、维管形成层的发生
　　(一)维管形成层的发生(来在茎的初生结构中，初生木质部和初生韧皮部之间保留有一层分生组织，即束中形成层(fascicular　ambium)。当茎开始次生生长时，在茎的初生结构的髓射线中，与束中形成层相对应的部位的薄壁细胞恢复分裂活动，发育为束间形成层(interfascicular cambium )( 图3-18)。束间形成层和束中形成层衔接起来成为一环,即维管形成层。

Similarities Between Fascicular and Interfascicular Cambium

Fascicular and interfascicular cambium are two types of vascular cambium tissues in the dicot stem.

+Both are made up of actively dividing meristematic cells.

+Besides, their main function is to produce vascular tissue.

+Also, they produce xylem towards the inner side of the stem while phloem towards the outer side.

Difference Between Fascicular Cambium and Interfascicular Cambium

Definition

Fascicular cambium refers to the cambium that develops within the vascular bundles in the stem of a plant while interfascicular cambium refers to the cambium arising between the vascular bundles in the stem of a plant. Thus, this is the main difference between fascicular cambium and interfascicular cambium.

Type of Meristem

Moreover, fascicular cambium can be both primary and secondary meristem while interfascicular cambium is a type of secondary meristem.

Derived from

Another difference between fascicular cambium and interfascicular cambium is that fascicular cambium is derived from the pro-meristem while interfascicular cambium is derived from the permanent tissues of the stem.

Occurrence

Also, while fascicular cambium occurs in both primary and secondary parts of the stem, interfascicular cambium only occurs in the secondary parts of the stem.

Function

Furthermore, fascicular cambium gives rise to both primary and secondary vascular components while interfascicular cambium gives rise to the secondary vascular components. Hence, this is the functional difference between fascicular cambium and interfascicular cambium.

Significance

Besides, an important difference between fascicular cambium and interfascicular cambium is their significance in the plant growth. The primary parts of the fascicular cambium are transformed into the secondary parts of the fascicular cambium while the interfascicular cambium fuses together with the fascicular cambium to form a continuous ring of meristematic tissue.

Conclusion

Fascicular cambium is a type of primary meristem that occurs between the xylem and phloem of a vascular bundle of the dicot stem. It is responsible for the formation of vascular tissues of the vascular bundle in the primary stem and then, it is transformed into the secondary meristem. In contrast, interfascicular cambium is a type of secondary meristem that occurs between two vascular bundles. Moreover, it arises from the permanent tissues of the stem and fuses with the fascicular cambium to form a continuous ring of vascular cambium. Therefore, the main difference between fascicular cambium and interfascicular cambium is the location in the stem and the type of growth.

(二)维管形成层的组成
维管形成层由纺锤状原始细胞(fusiform initial cell)和射线原始细胞(ray initial cell)两种组成(图3-19)。纺锤状原始细胞为长度超过宽度数十至数百倍的锐端细胞，细胞的切向面比径向面宽; 射线原始细胞为长形至近等径形。

Light microscopy of active (a, c) and dormant (b, d) cambium in P. tomentosa. a, b transverse section, c, d tangential section. Note the characteristic ''beaded'' appearance caused by wall thickening between primary pit fields in the radial walls (arrowheads). C cambium, P phloem, X xylem, V vessel, ri ray initials, fi fusiform initials. Bar 20 lm

(一)双子叶植物茎的次生结构

  1．双子叶植物木质茎的次生结构

  (1)维管形成层的发生及其活动：初生木质部与初生韧皮部之间保留的一层具分裂能力的原形成层细胞不断伸长．侵入生长形成_了形成层纺锤状原始细胞(fusiform  initial)．次生生长时部分纺锤状原始细胞横向分裂形成射线原始细胞(ray initial)，至此形成了束中形成层．当束中形成层开始活动后，邻接束中形成层的髓射线细胞脱落分化、恢复分生能力，形成束间形成层(interfas··icular carolfium)，并和束中形成层相互衔接．形成连续的形成层环(图—23)  维管形成层南纺锤状原始细胞和射线原始细胞组成．纺锤状原始细胞呈长梭形，两端尖，切向面较径向面宽，它与茎长轴平行排列，细胞质稀薄，液泡明显，核相对小，其分裂、分化可形成纤维、导管、管胞、筛胞、筛管和伴胞等，构成茎的轴向系统；射线原始细胞近等径，细胞小，数量少，其分裂、分化所产生射线细胞，构成茎的径向系统。纺锤状原始细胞主要进行平同分裂，分裂1次产，1i两个子细胞．1个向内分化|}I{次，t术质部J糸始细胞或向外分化…次生韧皮部原始细胞，，j 1个则保留为纺锤状原始细胞通常形成数个次生术质部细胞后才形成1个韧皮部细胞．N此次生木质部细胞数量明显多=次q三韧皮部细胞射线原始细胞也是以平周分裂为主．向内分裂产乍次乍术射线(xylem ray)和向外分裂产乍次乍韧皮射线，随攀的增半H，射线的数|_l也增加．两者合称维管射线(Va,"4．t·um ray)  随着次生术质部的较怏增加，形成层也进行垂fI啬!1分裂．增加本身的细胞，位置也逐渐向外推移  小本植物次生构造中维管束之间距离较仞生构造的窄：藤本植物次乍生长时．束问形成层不分化为维管组织．只分化}f-{簿罐细胞．所以其次生维管组织仍呈分离状态，束间距离较宽．如木通马兜铃、JII小通等．药材鉴别中称“车轮纹”：    

(2)次生木质部：由导管、管胞、木薄壁细胞、小纤维和木射线组成，导管主要为梯纹、网纹及孔纹导管，其中木本茎孔纹导管最普遍，草本茎以网纹及孔纹为主，、导管、管胞、木薄壁细胞和木纤维等是次生木质部中的纵向系统．木射线是其径向系统。木本植物茎中，次生木质部占有绝大部分，树木越大．次生木质郭占的比例也愈大．形成层的活动随季节变化表现出有规律的盛衰变化，因而季节的影响存结构上表现出明显的差异。因温带和亚热带春季或热带雨季气候温和，雨量充足，形成层活动旺盛，所形成的次生木质部细胞径大壁薄，质地较疏松，色泽较淡，称春材(spring wood)或早材(early wood)；温带夏未秋初或热带旱季，形成层活动逐渐减弱．所形成的木质部细胞径小壁厚．质地紧密、色泽较深．称秋材(autumn wood)或晚材(late wood)．横切面观一年中早材和晚材是渐变的．没有明显的界限，似当年的秋材与第二年的春材界限分明，形成一同心环层，称年轮(anntlal ring)或生长轮(growth ring)。但有的植物(如柑橘)一年可以形成3轮，这些同心环层称假年轮，这是因形成层有节奏地活动，每年有几个循环的结果：也可因一年中气候特殊变化．或树叶被害虫吃掉．生长受影响而引起假年轮的形成，在有干、湿季节的热带地区．树木也产生年轮状环层木质茎(木材)．横切面观，靠近形成层的部分颜色较浅．质地较松软。称边材(sap wood)，其含有生活细胞．具输导和贮藏作用。

而中心部分颜色较深，质地较坚同，称心材(heart wood)；因养料和氧不易进入．组织老化或衰f=，同时有些射线细胞或轴向薄壁细胞通过纹孔侵入导管或管胞腔内．膨大并沉积树胶、单宁、挥发油、色素等代谢产物．形成部分堵塞或完全堵塞导管或管胞腔的突起结构．称侵填体(tylosis)．使导管或管胞是失输导能力  心材比较坚硬，加强支持的作刖，不易腐烂，目．常含有多种代谢产物，，因此药川价值比边材高，如苏术、檀香、降香等均是心材入药(图2—26)。

    小质部各种组织纵横交错。十分复杂，要充分理解其结构．需从横切面、径向切面、切向切面进行比较观察，鉴定药材时常需观察i个切面的结构．

横切面(1rans忡佟P seclion)是与茎纵轴垂    罔2―26 iJq年qj小质攀(锻树)的构造

^所作的切面．可见年轮呈同心环状．射线呈辐射状排列，射线的长度和宽度：导管、管胞、木纤维和术簿壁细胞等呈类网形或多角形．

以及它们横切面的直径大小、细胞壁厚薄，胞腔大小等．

    径向切面(radial州，lion)是通过茎中心作的纵切面，可见年轮呈垂直平行的带状：射线横向分布并与年轮垂自．射线的高度和长度；导管、管胞、术纤维等旱纵长筒状或棱状．

以及它们纵切面的长度、直径、两端形状和次生壁的增厚纹理，    切向切面(tangential蝌・tion)是不通过茎中心而垂直于茎半径所作的纵切面，可见年轮呈“U”形波纹；射线为横切面，细胞群呈纺锤状，呈不连续的纵行排列．以及射线的高度、宽度、细胞列数和两端细胞的形状；导管、管胞、木纤维等的特征与径向切面卡H似    术材的i个切面中，射线的形状最突出，可作判断切面类型的依据(图2―27)：

    (3)次生韧皮部：由筛管、伴胞、韧皮纤维和韧皮薄壁细胞组成，韧皮射线的细胞壁不木质化，形状也不如木射线那样规则，其长短宽窄因植物种类而异。部分植物的次生韧皮部还有石细胞．如肉柞、厚朴、杜仲，也有的具乳汁管，如夹竹桃?次生韧皮部的输导功能通常仅有一年，到秋天就停止输导I斫死亡，但少数植物如葡萄则经过冬季的休眠．春季可重新恢复活动。，次生韧皮部形成时，初生韧皮部被挤J氍到外方。彤成颓废组织(obliterated tissue)，即筛管、伴胞及其他薄壁细胞被挤压破坏．细胞界限不清。次生韧皮部的溥壁细胞中含有多种营养物质、生理活性物质和次生代谢产物，具有一定的药用价值．如

Stem of a dicotyledon plant with secondary tissues. The pith has been compressed into the centre of the stem by the expanding secondary xylem. The secondary xylem now forms a complete ring, later the pith will be completely compressed and no longer be present.

The locations of the secondary xylem, vascular cambium, secondary phloem, cortex, cork cambium and bark. The interior of the stem is the left and the exterior is to the right.

Three year old woody stem of Tilia showing annual formation of xylem growth rings in the stem. Pith is still present in the middle of the stem.

Four year old woody stem of Liriodendron showing the pith in the centre and purple stained xylem tissue. Darker purple lines of cells are ray parenchyma.

Magnified view of the ray parenchyma. The lines of parenchyma cells continue out into the green stained phloem tissue. Xylem vessel cells are large at the beginning of the growth season and become smaller at the end of the growth season.

(4)木栓形成层的产生及其活动：

维管形成层活动的结果是次生维管组织不断增加．使茎不断增粗、而表皮一般不能分裂以适应这种增租，不久就被内部增长产生的压力挤碎失去保护作用。此时皮层外围或表皮细胞脱分化产生新的保护组织(图2-28）多数植物茎的木栓形成层由邻接表皮的皮层细胞恢复分裂机能而产生周皮，代替表皮行使保护作用，如杨树柳树、苹果等，也可来源于韧皮部薄腔细胞，如石栅数月、多数树本又依次在其内方而生新的木检形成展，形成新的周皮，如此其发生位置就会逐渐向内推移，可深达次生韧皮部、老及内方红织被新周皮丽高后逐渐枯死，这些周皮以及被它隔离死亡组织的综合体常白然刘路、敏称客皮赵hviome片状脱落，如白皮松。也有的星环状脱客如许精。有的裂成纵沟。

片脱落，如悬铃木:但也有的周皮不完落，如真建或外树皮，而林业和药材学则将树王王人新下的晓林树皮、如皮类药材基朴、杜仲、肉化，黄柏。秦皮。会欢皮的药用部分均浙该类树文周文形成时，他于原来气孔下方的木栓形成层较其他部位活跃，向外街生出种脑间聪发达的薄细胞，它们实破周皮，在树变表面形成此点线状突起或凹陷的结构，称定fmxel)道，皮孔的形状颜色和分布随植物种走而品恢复分裂能力，形成木栓形成层，分裂榆树:也有直接由表皮细胞产生。，如面布等，木栓形成层的活动常不过有的落皮层星鳞如柳、榆，有的呈大杜仲植物学中常将落皮层称树皮，史孔是周皮上气体交换的通建尖杰石材整定的依据之

一、茎的次生生长

二、茎的次生结构