88 BOTANY. [CHAP. n. 



able to procure their food from one particular kind of 

 host, it follows that its distribution is determined by that 

 of its host. Saprophytes, only requiring decomposing 

 organic matter, have a somewhat wider range, being 

 confined to those regions where life exists, but some 

 saprophytes appear to be partial to special kinds of 

 organic food. 



Metastasis. The substance formed by assimilation in 

 the chlorophyll-grains, which is usually starch, sometimes 

 fat, or rarely cane-sugar; this constitutes the raw material 

 from which all the substances present in plant tissues 

 are formed. In this elaboration, oxygen, nitrogen, and 

 the various mineral substances taken from the soil are 

 also utilized. The most important and indispensable of 

 substances formed by the plant are those concerned in 

 the formation of the protoplasm and the cell-wall ; such 

 are called plastic substances. The plastic substances are 

 not used up entirely in the organs where they are formed, 

 but change their position and are either used up at once 

 in the formation of new cells, or are stored up as reserve 

 material for future use. The change in position of 

 elaborated material is usually accompanied by a change 

 in chemical composition for the purpose of facilitating 

 the transport. This is the process of Metastasis or Meta- 

 bolism. The solid starch formed in chlorophyll-grains is 

 converted into a glucoside,in which condition it can travel 

 along the stem, and is converted back into starch when 

 it reaches the specialized part destined to store it up 

 until required for use ; all seeds contain a supply of 

 reserve material for the purpose of furnishing the young 

 plantlet with its first food until it develops green leaves 

 and is able to assimilate for itself. Potato tubers, 



