GROWTH 



The proembryo cell divides in several planes, the number of 

 cells and the size of the embryo plant increasing in the process. 

 Some of the cells so produced form seed leaves, some a young 

 stem, and some the beginnings of a root, the whole comprising 



an embryo plant which lies 

 within the seed. Both increase 

 in size and differentiation (in- 

 crease in complexity) occur in 

 this period of growth from zy- 

 gote to embryo plant. 



If we plant the seed it ab- 

 sorbs water and the food ma- 

 terials stored in the seed are 

 digested. The cells of the em- 

 bryo plant increase in size 

 and number and the seedling 

 bursts its way out of the seed, 

 establishing its root system in 

 the soil and its stem and leaves 

 in the air. From this time on 

 the growth is largely limited 

 to particular parts of the plant 

 body. A region consisting of a 

 group of cells at the end of the 

 main stem and of each branch 

 and at the tip of each root and 

 rootlet, grows. The growth of 

 these regions results in increase 

 in length and in the formation 

 of leaves, flowers, fruits, and 

 seeds. In addition, in plants such as our trees and many of our 

 common herbaceous plants, a cylinder of tissue, the cambium, 

 found between the wood and the bark, retains the power of 

 growth. The growth of the cambium results in increase in di- 

 ameter. A thin slice or section of the growing region of the root 



Figure 



of 



seed plant, 

 The single- 



Early growth 

 the peppergrass {Lepidium). 

 celled zygote (g). B. The zygote has grown 

 into a chain of cells, s is the suspensor and 

 e the proembryo cell. C, D, and E. The 

 further development of the proembryo cell 

 into an embryo plant. The beginnings of 

 two seed leaves are evident in E. F. A still 

 later stage, the cells are not shown. Note 

 the seed leaves (c) the stem (st) and root 

 O). From Curtis, Nature and Development 

 of plants. By permission of Henry Holt and 

 Company. 



