•750 PLANT GROWTH lO 



wide, in the form of a long main "stem" with whorls of lateral (unicellular) branches. 

 Here enlargement is not so localized but a whole series of "stem" and branch 

 cells may enlarge together in length and girth (Green and Chapman, 1955). 



The other two differences from animal growth are at the cellular level, namely 

 the rigid cell wall and the relatively large vacuole. Both have a marked influence 

 on the growth process. 



The rigid, elastic cell wall is characteristic of the vegetable kingdom from flowering 

 plants to bacteria. Exceptions are found in the fungal group Myxomycetes (sometimes 

 classed with the animals as Mycetozoa), in one group of .4/^^^ (the Eugleninae or 

 Euolenophyta) , and in the Myxobacteria and the Spirochaetes, among the bacteria. 

 These have either flexible walls or a membrane around the cytoplasm (Thimann, 

 1955' Chaps. 2 and 3). All other plants have the characteristic rigid cellwalls, 

 formed of various mixtures of polysaccharides and polyuronides. In most fungi 

 other than some of the aquatic Phycomycetes the wall is in part chitin (a polymer of 

 N-acetyl-2-glucosamine) together with varying amounts of glucose polymers, 

 though in yeasts the chitin appears to be present only in traces. In some algae, 

 like the diatoms, the wall is heavily impregnated with silica and as a result the 

 cells cannot enlarge at all but have to go through an occasional brief naked stage. 

 In all land plants so far as known, in the green algae and in the Oomycetes among 

 the fungi, the wall contains cellulose, the toughest and most inert of the fiber- 

 materials formed out of polysaccharides. 



Cell-walls of the land plants are divided into primary and secondary, according to 

 whether they can still extend or not. Secondary walls are usually thickened by 

 successive depositions of material from within. An example is given by the cells of 

 wood, which are highly encrusted with lignin, interpenetrating the cellulose and 

 other materials, and often filling up most of the lumen of the cell. Such cells are 

 almost invariably dead. The elongated cell which comprises a cotton hair, on the 

 other hand, has its secondary wall made up of almost pure a-cellulose, which 

 gives it its great tensile strength. Secondary walls are of most interest and impor- 

 tance for fiber and textile technology, but, from the viewpoint of growth studies, it 

 is the primary walls that are of most interest. Analyses of the primary walls of the 

 first shoot or coleoptile of corn {^ea mays) and oats [Avena saliva) and of the 

 growing parts of two other seedhngs are collected in Table i . The data must be 

 considered quite rough since different investigators have used different material 

 and different methods. It will be noted, however, that the cellulose, important as 

 it is in principle, is not necessarily the largest fraction of the cellwall. Its role in 

 growth will be taken up in section VI (p. 775). 



The vacuole comprises a relatively thin liquid phase separated from the cyto- 

 plasm by a flexible membrane, the tonoplast. In small meristematic cells the 

 vacuoles are numerous and minute filaments or globules, which may be really 

 fragments of vacuolar membrane (tonoplast) with httle or no liquid content. As 

 the cell enlarges these primordia increase in volume and flow together, eventually 

 forming a sap cavity which occupies most of the cell volume (Fig. i). In some 

 cells, of which the stamen hairs of Tradescantia are a classical example, cytoplasmic 

 threads penetrate the vacuole in many directions and the nucleus lies suspended 

 in the center surrounded by a layer of cytoplasm and connected to the peripheral 



