1 8 THE PHYSIOLOGY OF PLANTS 



substances, which may now make their appearance. Thus we 

 often find in these cells a certain amount of starch, which 

 makes its way down from the upper portions of the plant, and 

 may be looked upon as a reserve substance, which the plant is 

 able to store up, if the leaves are producing more of it than 

 can at the time be used up. In cells belonging to the upper 

 portions of the plant, which afterwards become green, the green 

 corpuscles {chlorophyll corpuscles) make their appearance in 

 the primordial utricle, as may be seen from Fig. 6 d. 



In consequence of the storage of starch grains within the 

 cells, the cortex of some roots — the carrot, for instance — in 

 the autumn has a snowy white appearance and is full of starch, 

 so that we may look upon the cortex of the root as a storage 

 tissue. This storage is sometimes only of a transitory nature, 

 often limited to a single layer of cells, which surrounds the 

 vascular cylinder. In such a case this layer of cells is termed 

 a stareh-sheath, or, if it contains sugar, a sugar-sheath. 



The function of the cortical cells as a storage tissue is chiefly 

 seen in that portion of the root where root-hairs have already 

 disappeared and the epidermis has become thick-walled and 

 has lost its absorptive power. Hence we can divide every 

 root functionally into three zones of very unequal lengths. 

 The apex, protected by the root-cap, is the region of cell-division, 

 and is concerned in the formation of new tissues. Then we 

 gradually pass into the absopptive region, which is also the region 

 of cell-growth. Here all the cells increase in size, especially in 

 length, and this region is consequently the chief seat of the 

 elongation of the root. A number of the epidermal cells, too, 

 grow out into root-hairs, which undertake the absorptive 

 function. The absorbed salts and water pass on through the 

 young cortical cells into the vascular cylinder, and through the- 

 vessels up into the leaves of the plant. Behind the absorptive 

 region begins the third zone, which reaches back to the insertion 

 of the root, and which comprises the fully developed portion of 

 the root. This older region of the root has now lost its absorp- 

 tive powers, and the parenchymatous cells of the root serve 

 only as a storage tissue for reserve substances, which are of the 

 nature of organic substances, and which have been elaborated 

 in the leaves and have passed down to the root structures. 



