AN IXDIN'IUrAL PLANT IN ALL OF ITS RELATIONS 1'69 



miicli mure iufluouced in direction li\' otlier external 

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

 the soil is not i)erfectly uniform, and contact witli different 

 substances induces curvatures, and as a result of tliese and 

 otlier causes, the root system may become very intricate, 

 wliicli is extremely fa\or- 

 al)le for aljsorliiiig and \\^ 

 gripping. 



OCi. Direction of the stem. 

 — As soon as the stem ti]) 

 is extricated from the seed, 

 it exhibits sensitiveness to 

 the light intluence {Jieliot- 

 ropisin), l)eing guided in 

 a general way towards the 

 light (see Fig. 14;j«). 

 Pirectioii towards the 

 light, the source of the in- 

 fluence, is spoken of as 

 posifire heliotropism, as 

 distinguished from direc- 

 tion aAvay fr(]m the light, 

 called rirgativc heliotro- 

 jHsm. If the main axis 

 continues to develo}). it 

 continues to show this posi- 

 tive helioti'opism stronglv, 

 1,iut the branches ma}' sliow 

 every variation from positive to transverse lieliotropism ; 

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

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

 ners, etc., show strong transverse heliotropism. while otlier 

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

 ropism. 



07. Direction of foliage leaves. — The general direction of 

 foliage leaves on an erect stem is transversely heliotro2'>ic ; 



Fig. 14.3. Germination of tlie si'wl of 

 arljor-vitie {'J'liujui. 1: slious tlic 

 emOTgciicc- of the a.\is (rl which is to 

 develop the root, and its turning to- 

 wards tlie soil, r shows a later stage, 

 ill wliieli tile root ,/l has heen some- 

 uliat develo]ieil, and tile slem of the 

 embryo {]t} is devilii[>inL' a curve pre- 

 paratory to pulling out tile seed lea\es 

 (eotyiedons). E shows the young plant- 

 let entirely fii-e fioni the seed, with its 

 root {/■) extending into tlie soil, its stem 

 (/() erect, and its Jirst leaves (f;i hori- 

 zontally spread.— After STiiisEfiaiER. 



