238 CAUSES OF VARIATION 



A stem of willow severed from its parent plant and suspended, 

 apex upward, in a moist atmosphere, will of course send out 

 shoots from its apex and roots from its base. If a ring of bark 

 be removed from the middle of the stem, then sprouts will issue 

 from the apical extremities of the sections and roots from the 

 basal end. Neither of these experiments determines whether 

 gravity or polarity is chiefly instrumental in the production of 

 stem and root, but if the piece be inverted and suspended apex 

 downward (in a moist atmosphere), we shall get some light on 

 the two forces, external and internal. 



Under these conditions the apical end, now downward, will 

 yet produce stems, but they will change their direction with ref- 

 erence to the axis and point upward, while the basal end will 

 produce roots, but they will extend downward. In this case 

 each end has produced its characteristic growth, and each has 

 responded to gravity in the usual way (see Fig. 26), except that, 

 if the piece be of the older wood, roots will appear throughout 

 the entire length. The force that fixes the character of growth 

 appears to be internal ; that which fixes its direction appears to 

 be mainly external. 



If a begonia leaf be planted in the ground or suspended in 

 moist air, whatever its position roots will start from the basal 

 end of the stem at its point of severance, and afterward shoots 

 will arise just above the point of origin of the roots, the body of 

 the leaf withering away. 1 (By "above" is meant between the 

 origin of the root and the apex of the leaf, whatever its position.) 



By this we see that the stem has a distinct polarity, producing 

 sprout and root, each at the proper end ; that the leaf has no 

 true polarity, producing primarily only roots ; but that wherever 

 and however produced, a distinct geotropism characterizes both 

 stem and root. 



There is little geotropism in leaves, or in the horizontal stems 

 of many plants running along or just below the surface of the 

 ground. However, the stems and roots produced 'at the nodes 

 of these underground stems are both geotropic. 



Geotropism in animals. Geotropism is much less marked with 

 animals than with plants. We may say that only fixed organisms 



1 Morgan, Regeneration, pp. 74-76. 



