DIEECTION OF TEaETABLE GEOWTH. 375 



weight of tte liquid in the cells above them. In other 

 words, they are subject not only to the equal hydraulic 

 pressure originating in the roots, but also to a slight hy- 

 drostatic pressure from the overlying cells. This pro- 

 duces the greater extension of the lower passive tissues, 

 and aocounts for the curvature upward. When the stem 

 becomes vertical the hydrostatic pressure is equal on both 

 sides of the stem, and the latter is accordingly maint^in^d 

 in that position. (Hofmeister, Sachs.) 



Effect of Light. — Besides the influence of gravitation 

 and of interior tension, that of the solar light must be re- 

 garded, as it assists largely in producing the more com- 

 plex phenomena of direction in the growth of plants. 

 The explanations already given refer to the plant when 

 unaffected by light. As is well known, the stems, leaves 

 and roots of plants, when growing where they are un- 

 equally illuminated, as in a window, in most cases curve or 

 turn towards the light. More rarely is curvature away 

 from the light observed, as in case of the stems of ivy, 

 (Hedera helix), and the young rootlets of the mistletoe, 

 ( Viscum album). The common nasturtium, {Tropmolum 

 Tnajus), exhibits in its young stems inclination towards, 

 in its older stems inclination away from, the light. Its 

 leaves turn always towards, its roots growing in water 

 often curve towards, often away from the light. 



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