108 HISTOLOGY AND PIIYSIOLOGT. 



it, therefore, consumes oxygen instead of liberating it, as 

 in assimilation. Instead of increasing the weight of the 

 plant, as is the case in assimilation, it decreases it ; it takes 

 place in all cells, and in darkness as well as in sunlight. At 

 the expense of the elaborated materials, — called formative 

 material, — growth of tissue, that is, the multiplication of 

 cells, takes place. This may occur at once, or a portion of 

 the formative materials may be stored up for subsequent 

 use. Thus, in case of the Potato-plant, starch, which is 

 formed in the chlorophyll grains in sunlight, becomes 

 dissolved when darkness sets in, and is transported through 

 the parenchyma and thin-walled cells- of the phloem to the 

 reservoir of reserve material. The tuber, bulb, rhizome, 

 stem, seeds, and- spores hold in store varying amounts of 

 reserve material. 



138, This reserve material furnishes the elaborated 

 food for the beginning of the subsequent period of growth ; 

 thus, from that stored up in stems and roots, the buds 

 develop in the spring ; from that in the bulb, a cluster of 

 flowers may form ; from that in the tuber, the " eyes" 

 (buds) develop into branches ; from that in the seed, the 

 embryo unfolds into a self-supporting plantlet. The reser- 

 voirs are emptied gradually as the growth of the new 

 organs progresses. When they are entirely empty, growth 

 ceases, unless the conditions of assimilation (presence of 

 chlorophyll and sunlight) render the formation of new 

 organic material possible. The direction of the transport 

 of assimilated materials is from the assimilating organs 

 directly to the growing parts, or to the reservoir of reserve 

 materials, thence to the growing parts. The movement is 

 mainly one of diffusion. "The pressure caused by the 

 tension and turgescence of the tissues has, in addition, a 



