Physiological Morphology 89 



What circumstances had all these experiments in connnon ? 

 The stems always originated from the uj^ixt end or side of an 

 element, and roots always from the lower side or end uf the same 

 element. These facts can be explained only on the assump- 

 tion that in this case gravitation determines the place of origin 

 of organs. 



Now we may ask whether the action of this force, gravita- 

 tion, is also responsible for the natural arrangement of the organs 

 in this form, namely, that roots appear only at the base of the 

 stem and never at the apex or in the place of a branch. I believe 

 that this is the case. By reason of its negative geotroi)ism, the 

 stem grows vertically upward. Gravitation does not p<Tmit 

 roots to arise at any place except at the under side of the 

 organs, and that is, under normal conditions, at the base of the 

 stem. The same force determines that pol^-ps can originate 

 only on the upper side of branches, and thus the general 

 arrangement of organs is brought about by gravitation. ]iut 

 how does gravitation determine that steins grow on tlie upjKT 

 and roots at the under side? This is a question to which we 

 shall return later. 



Fig. 27 gives a drawing of an example of heteromorphosis 

 in Margelis, a hydroid common at Woods Hole, upon which 

 another set of experiments was carried on. If we cut off a stem, 

 or a small piece of a stem of this hydroid, and place it in a dish 

 containing sea-water, protecting 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 give ri.se 

 to roots that spread out and very soon cover a large area of the 

 glass. In this way the apical end of a stem may continue to 

 grow as a totally different organ, namely, as a root. Every 

 organ not in contact with some solid body gives rise to pol>7)s. 

 Even the main root, if not in contact with a solid body, no longer 

 grows as a root, but gives rise to a great number of small i)olyps 

 which appear at the end of long stems. Fig. 27, which Mr. 



