56 Growth 



If a plant is to grow, this must be accomplished by allowing some of 

 its cells to escape the general fate and remain capable of division, pro- 

 gressively forming new tissues but preserving a remnant that persists in 

 a perpetually embryonic condition. The plant body grows in size by the 

 activity of such localized growing points or regions, the meristems, which 

 are centers of cell division and cell expansion. 



The axis of the plant grows in length by a meristem at the apex of stem 

 and root, and in width by a sheath of lateral meristem, or cambium. De- 

 terminate organs such as leaves, however, rarely have sharply localized 

 meristems but enlarge throughout much of their extent, as an animal body 

 does, until growth ceases. Meristems are obviously of much interest to 

 the student of plant morphogenesis. They provide, in a sense, a con- 

 tinuous embryology for the plant and offer an important point of attack 

 on the problems of plant development. 



APICAL MERISTEMS 



In the simplest plants, the lower algae and fungi, growth is hardly 

 localized at all. Cells capable of division are either present throughout 

 the plant body or in considerable portions of it, and nothing which might 

 be called a meristem is to be found. In Spirogyra, for example, growth 

 in length of the filament is produced by cell division almost anywhere in 

 it. In such a membranous type as Ulva, growth results from divisions at 

 right angles to the surface throughout most of its area. In coenocytic 

 forms, the whole thallus enlarges, and growth is not related to cell di- 

 vision at all. 



In some of the simpler filamentous brown algae, however, growth in 

 length is limited to the tip of the filament, which is occupied by a single 

 large cell. This divides transversely, and a series of daughter cells is thus 

 produced from its basal face. They and their daughter cells divide a few 

 more times, but division finally ceases. The only permanently embryonic 

 cell is the apical one, which thus dominates the development of the plant 

 body. Branches originate by the lengthwise division of this apical cell 

 (Fig. 4-1). 



In types like Fucus, with larger and more complex plant bodies, growth 

 still originates by the activity of an apical cell, which occupies the base 

 of a terminal notch in the thallus. This cell cuts off daughter cells from 

 its two lateral faces. From these and their descendants are formed the i 

 various tissues of the thallus. A fern prothallium grows in much the same 

 way, developing under the control of the meristematic region in the 

 notch. This control may be relaxed, however, and almost any cell in the 

 structure may begin to divide. Many prothallia never form the typical 

 heart-shaped structure. 



