33° 



LECTURE XX. 



of unemployed plastic substances taking place. In most cases, however, and particularly 

 towards the conclusion of the period of vegetation, the quantities of such materials 

 used up for the purposes of growth are smaller than those produced by assimilation. 

 The superfluous products of metabolism thus arising are then stored up in the tissues 

 of the plant, to be preserved until the beginning of the next period of vegetation. 

 In this condition they are termed reserve-materials, a name which was introduced by 

 Theodore Hartig some years ago. The reserve-materials may be accumulated in 

 the parenchyma and phloe'm of all ordinary perennial organs, particularly in trees 

 and other woody plants ; the twigs, branches, roots and stems of which have their 

 cortex and the parenchymatous cells of the alburnum filled during the summer and 



Fig. 226.— a Potato plant developed from seed, r r the roots j c cotyledons ; J"^ leaves ; b b subterranean shoots 

 which produce the tubers lb (after Duchartre). 



autumn with reserve-materials, which are used up when the buds put forth shoots 

 in the spring. If the formation of seeds takes place at the end of the period of vege- 

 tation, large quantities of reserve-materials are accumulated to a remarkable degree in 

 these, all non-plastic compounds being excluded ; and in annual plants the seeds are 

 in fact the only reservoirs of reserve-materials. In all perennial plants, however, 

 it is chiefly the subterranean_root-stocks, rhizomes, bulbs, and tubers, etc., which 

 especially fulfil the function of storing the substances assimilated during the period of 

 vegetation ; and on the formation of the germinal shoots and roots at the beginning 

 of the next period of vegetation, they yield them up to these growing parts. In general, 



