AN INDIVIDUAL PLANT IN ALL OF ITS RELATIONS. 139 



much more influenced in direction by other external 

 causes, especially the presence of moisture. As a rule, 

 the soil is not perfectly uniform, and contact with different 

 substances induces curvatures, and as a result of these and 

 other causes, the root system may become very intricate, 

 which is extremely favor- 

 able for absorbing and 

 gripping. 



96. Direction of the stem. 

 — As soon as the stem tip 

 is extricated from the seed, 

 it shows a response to the 

 light influence {heliotro- 

 pism), being guided in a 

 general way towards the 

 light (see Fig. 143a). 

 Direction towards the 

 light, the source of the in- 

 fluence, is spoken of as 

 positive heliotropism, as 

 distinguished from direc- 

 tion away from the light, 

 called negative heliotro- 

 pism. If the main axis 

 continues to develop, it 

 continues to show this posi- 

 tive heliotropism strongly, 

 but the branches may show 

 every variation from positive to transverse heliotropism ; 

 that is, a direction transverse to the direction of the rays 

 of light. In some plants certain stems, as stolons, run- 

 eta, show strong transverse heliotropism, while other 

 stems, as rootstocks, etc., show a strong transverse geot- 

 ropism. 



Direction of foliage leaves. — The general direction of 

 foliage leaves on an erect stem is transversely heliotropic ; 



Fig. 143. Germination of the seed of 

 arbor- vifce (Thuja). B shows the 

 emergence of the axis (r) which is to 

 develop the root, and its turning to- 

 wards the soil. C shows a later stage, 

 in which the root (r) has been some- 

 what developed, and the stem of the 

 embryo (A) is developing a curve pre- 

 paratory to palling out the seed leaves 

 (cotyledons). E shows the yonng plant- 

 let entirely free from the seed, with its 

 root (r) extending into the soil, its stem 

 (A) erect, and its first leaves (c) hori- 

 zontally spread.— After STRASBrRGER. 



