THE GENERAL CHEMISTRY AND PHYSICS OF PLANT-LIFE. 263 



assume that the cell-walls are more expansible in the transverse 

 direction ; that is, the individual cells of the root-parenchyma be- 

 come shortened and much expanded laterally. The root-cortex 

 remains passive during this process of shortening and becomes 

 transversely wrinkled. The purpose of such shortening of roots is 

 quite evident. For example, a rosette of radical leaves (cataphyi- 

 lary) which, according to their structure and for mechanical rea- 

 sons, should remain near the ground will thereby remain in the 

 same position, although a short portion of the stem below the leaves 

 elongates somewhat. Such shortening also tends to hold the plants 

 more firmly in the soil. This phenomenon received a special sig- 

 nificance from the observations made by Fittmann (1819). The 

 winter-buds of biennial plants whose cotyledons and plumules are 

 already above the soil may withdraw them into the soil on the 

 approach of winter. H. de Yries has explained the mechanics of 

 this phenomenon. 



As a second result of unequal growth within an organ we may 

 mention 



(h) Curvatures. 



Curvatures of cylindrical or prismatic organs ^ take place when 

 any given longitudinal portion of tissue elongates or shortens more 

 than does a similar portion on the opposite side. It is evident that 

 all such curvatures are not dependent upon processes of growth, 

 since shortening may also be due to evaporation of water, and elon- 

 gation may be due to an increase in the amount of water taken up. 

 Such curvatures do not come under the category of " unequal 

 growth." Curvatures due to unequal growth are, however, of very 

 frequent occurrence. 



Leaving out of consideration the frequently occurring foldings 

 of cell-walls, as, for example, the "wavy " epidermis, the anatropous 

 seed-buds, etc., we shall refer more particularly to those curvatures 

 caused by one-sided exposure to light (heliotropism), by the one- 

 sided action of gravity (geotropism), or moisture (hydrotrojiism). 

 These are conditions which influence the growth of plants in a high 

 degree. The significance of the curvatures mentioned above is very 

 evident when we study the normal assimilating organs, the roots, 

 etc. The leaves must assume a position most favorable to the influ- 



' See Nageli and Schwekdener's Mikroskop. 



