234 BOTANY part i 



tion ; it is also able to pass from the protoplasm to the cell-sap or to the 

 cell-Avall, its place being taken by Avater which passes in from adjoining 

 structures. This water is not a constituent of the plant in the same 

 sense as the carbohydrates or proteids formed by assimilation are. It 

 is different where the water is chemically combined. This necessarily 

 takes place when carbohydrates are formed from carbon dioxide and 

 probably in other cases also. In these cases there is the same justi- 

 fication for speaking of the assimilation of the water as of the 

 assimilation of carbon dioxide. 



IV. Translocation and Transformation of Assimilates 



The assimilates serve primarily for the construction of new 

 substance of the plant and the growth of new cells. It is only rarely, 

 however, that growth takes place where the work of assimilation is 

 eftected. Thus, the assimilation of carbon dioxide goes on mainly in 

 fully-grown foliage leaves while the growing-points are more or 

 less distant from the leaves. The assimilatory activity and the 

 formation of new organs also do not coincide in time. Many 

 plants have periods of active assimilation when but little growth is 

 taking place and, alternating with these, periods of active growth 

 associated with little or no assimilatory activity. Our trees lose their 

 leaves in autumn and hei'baceous plants lose all the above-ground 

 organs. In both cases new organs of assimilation must be formed in 

 spring before assimilation can be resumed ; in the growth of these 

 organs the plant utilises stored assimilates. Every germinating seed- 

 ling also lives at first wholly at the expense of assimilates of a preced- 

 ing generation. Such stored -up assimilates are termed reserve 

 MATERIALS ; they may be deposited where they are formed or may be 

 carried to secondary places of deposit. Every foliage leaf which in 

 the evening of a bright summer's day is gorged with starch is an 

 illustration of the first condition. The second is seen in seeds Avhere 

 reserve materials are stored in the endosperm or the cotyledons. It 

 is also found in vegetative organs, which may even show by their form 

 that they are places for storage of reserve materials ; examples of 

 these are the swollen leaves of bulbs, swollen stems {e.g. potato) or 

 swollen roots {e.g. turnip). In order that assimilates should reach 

 these storage places they must be capable of TRANSLOCATION, and they 

 have also to be conveyed through the plant when they are removed 

 from the place of storage and employed in the development of new 

 organs. Many reserve materials or assimilates occur in a solid form 

 which does not allow them to pass from cell to cell ; starch is an 

 example of this. Others are, it is true, soluble, but have such large 

 molecules that they only diffuse with difficulty. For these reasons 

 reserve substances have usually to undergo a change before they can 

 be conveyed through the plant. 



