18 THU PLANT CELL. 



granule, where only the thinnest film of the substance of the 

 chloroplast remains (see Fig. 3). 



In Begonia leaf (cells of the mesophyll) starch-gi-anules are 

 formed at first in the interior of chloroplasts, but subsequent 

 growth proceeds at the side of these structures ; in Fallisneria, on 

 the other hand, the granules are seen to be centrally situated 

 from beginning to end. Moreover, even in the apparently fully- 

 formed grains, a delicate film of chloroplast substance is always 

 to be detected, stretched over the gi'ain. 



In this formation of starch in the chloroplasts of cells from 

 the green parts of plants is to be found a partial demonstration 

 of assimilation ; for a chloroplast is able, by means of its chloro- 

 phyll, to utilise during the daytime certain of the rays of white 

 light falling upon the leaves, these rays being turned to account 

 in the decomposition of the carbon dioxide which enters the cells 

 after having gained admission through certain pores (stomata) 

 existing in the epidermis (photosynthesis). In the substance of 

 the chloroplast certain somewhat complex chemical reactions take 

 place which result in the formation of carbon compounds, such as 

 starch, sugar, or cellulose from the carbon dioxide and water 

 supplied ; and in this process oxygen is evolved and passes out 

 again through the stomata. 



The whole process above described is, correctly speaking, only 

 part of the assimilatory reactions taking place in the cell ; for, as 

 will be pointed out more fully later on, nitrogenous substances 

 are also elaborated and assimilated in the leaf-cells, and the 

 materials resulting from this elaboration (amido-acids) are made 

 use of by the protoplasm in the complex processes involved in 

 formation of fresh protoplasm and nutrition of the cell as a whole. 

 Nevertheless, this preliminary study of the assimilation of carbon 

 dioxide and water in the chloroplasts, with the. optical demonstra- 

 tion of the final result — viz., formation of starch granules — is a 

 useful introduction to the investigation of other and possibl)! 

 more complex vital processes taking place in the cell (see Chap, x.)! 



B. The Young Undifferentiated Cell. 



A cell, such as the assimilating cell of Vallisneria leaf, does not, 



present the same features throughout its whole existence viz.j 



peripheral protoplasm, chloroplasts, and the phenomenon oi 



