CHAPTER VIII 



THE COLOURLESS PLANT CELL. THE YEAST 



PLANT 



WE have already seen that many of the living cells 

 of the higher green plants do not contain chlorophyll, 

 e.g. the meristematic cells, all the cells of the root, 

 many of those in the interior of the stem, and also 

 those which form the surface layer (epidermis) of her- 

 baceous stems and leaves. The cytoplasm of many of 

 these cells, however, contain plastids, which may turn 

 green on exposure to bright light, and thus become 

 chloroplasts. All these colourless cells have to be fed 

 with sugar, ultimately coming from the green cells, 

 to repair the waste of respiration ; and the meriste- 

 matic cells, which are actively dividing, not only 

 require large quantities of sugar, but must also be 

 supplied with nitrogenous substances to make new 

 protoplasm. 



There are also plants which have no green cells : 

 a few saprophytes and parasites J among the seed plants 

 which live upon the organic substances of humus 

 (decaying vegetable substance such as leaf mould) 

 or upon the organic substances of the bodies of living 

 green plants ; the great group of FUNGI ; and finally 

 the group of unicellular plants known as BACTERIA, 

 which, as we shall see in the next chapter, are of extra- 

 ordinary importance in the economy of the life of the 



' See p. 300. 



127 



