230 - Multicellular Plants 



12-33); and in the stem, there is a growing 

 point at the apex of each bud. However, 

 many stems (Gymnospermae and dicotyle- 

 donous Angiospermae) possess a thin layer 

 of meristem, the cambium, which encircles 

 the trunk and its branches, underneath the 

 bark. The cambium provides for increases in 

 the girth of the stem. Also in most roots (Fig. 

 13-11) a layer of meristem, the pericycle, re- 

 mains behind the growing point, providing 

 for the origin of secondary offshoots from 

 the primary root. 



The nature of growth and development in 

 plants is well illustrated by the case of a 

 rootlet (Fig. 12-33). Virtually all new cells, 

 both below and above the growing point, 

 are formed by the multiplication of the meri- 

 stem cells of the growing point. Below the 

 growing point, the new cells become differ- 

 entiated into the cells of the root cap. The 



VASCUUR TISSUE 



ROOT HAIR 



EPIDERMIS 



MATURE PART 



GROWING POINT 



ROOT CAP 



Fig. 12-33. Longitudinal section of a young root of 

 barley. 



root cap never becomes very large, however, 

 because the cells keep scuffing off as the root 

 pushes down into the soil (Fig. 12-33). Above 

 the growing point, the cells increase in size 

 (particularly in length), and become differ- 

 entiated into the several kinds of specialized 

 tissues which make up the body of the root. 

 Such mature differentiated cells develop thick 

 woody walls and become incapable of fur- 

 ther growth and division. 



In plants, growth tends to be localized and 

 potentially unlimited; while in animals, 

 growth occurs throughout the body and is 

 more limited in scope. Most animals stop 

 growing when they achieve a certain size 

 and form, but the size and form of a 

 plant are limited mainly by external condi- 

 tions. 



Embryonic Development in a Dicotyle- 

 donous Seed Plant. After fertilization, the 

 zygote begins development by a series of cell 

 divisions. These first few divisions are all in 

 one plane, producing a filament of cells, the 

 suspensor, which pushes along the main axis 

 toward the center of the endosperm tissue 

 (Fig. 12-34). The end cell of the filament 

 begins to divide in other planes, forming a 

 rounded mass of cells (Fig. 12-34). The whole 

 embryo is formed from this rounded mass of 

 cells, anil the suspensor degenerates before 

 the seed is ripe. Growth and cell division 

 now become most rapid at the sides of the 

 embryo, near the free end. This results in 

 two large outgrowths, which become the 

 cotyledons. Meanwhile one end of the em- 

 bryo elongates to form the hypocotyl, and the 

 epicotyl begins to appear in the notch be- 

 tween the extended cotyledons. In some 

 plants the embryo stops growing before it 

 occupies the whole seed — in which case an 

 endosperm persists around the embryo (Fig. 

 12-23). But in the bean (Fig. 12-21) and most 

 other Dicotyledoneae, the cotyledons con- 

 tinue to grow, absorbing all the substance of 

 the endosperm before the seed is ripe. In any 

 event, the integuments of the ovule finally 

 become thicker and tougher, forming the 

 seed coats. 



