40 



BIOLOGICAL LECTURES. 



position (Fig. 6), with the root end above, the branches on the 

 under side grow out as roots and at the upper end, a stem 

 arises as usual. 



What circumstances have all these experiments in common? 

 These two : stems always originate from the upper end or side 

 of an element, and roots always originate from the lower side 

 or end of the same element. These facts can be explained 

 only through the assumption that gravitation, in this case, 

 determines the place of origin of organs. 



Now we may ask whether the 

 action of this force, gravitation, 

 produces the natural arrangements 

 of parts, /. i\, roots growing only at 

 the base of the stem and never at 

 the apex or in the place of a branch. 

 I think that it does. By reason of 

 its negative geotropism, the stem 

 grows vertically upwards. Gravita- 

 tion does not permit roots to arise 

 at an)- place except the under side 

 of the organs, and that, in the 

 normal position, is the base of the 

 stem. The same force determines 

 that polyps can originate only at 

 the upper side of branches, and so 

 the main arrangement of organs is 

 brought about by gravitation. But 

 how does gravitation determine that 

 roots grow at the upper and stems 

 This is a question to which I shall return 



Fic. 7. 



at the under side 1 

 later. 



Fig. 7 is a drawing of a case of heteromorphosis in Margeliss, 

 a hydroid common at Woods Holl. 



If we cut off a stem, or a small piece of a stem, and place it 

 in a dish containing sea water, and protect it carefully from 

 every motion, a curious change takes place in the organism. 

 Almost all, and in some cases all of the stems which touch 

 the glass begin to give rise to roots that spread out and very 



