376 EFFECT OF MOLAR AGENTS UPON GROWTH [Ch. XIV 



horizontal slit was made in the side of the body-wall. The 

 tentacles over the slit contracted — there was a sort of negative 

 growth. The shortening was doubtless due, 

 as LoEB says, to the loss of water and con- 

 sequently of turgescence in this part of the 

 body-wall (Fig. 101). I have cut the body- 

 wall of a hydroid immediately below an 

 incipient bud, whereupon the bud at once 

 flattened out. These experiments show 

 how important water pressure is for the 

 maintenance of the size of the body and for 

 growth, and in so far explain the mechanical 

 effect of a cut or other similar wound. 



V_y § 2. Effect of Contact upon the Direc- 



tion OF Growth — Thigmotropism 



Fig. 101. — Cerianthua, 

 from which a piece, 

 a, b, c, has been cut, 

 causing a loss of tur- 

 gescence and conse- 

 quent shrinking of 

 tentacles on the cut 

 side. (From Loeb, 

 '92.) 



Having seen that molar agents can affect 

 the rate of growth, we are in a position to 

 understand how a molar agent, acting upon 

 one side only of an elongated organ or 

 plate of tissue, may induce a less or 

 greater growth upon that side, and, con- 

 sequently, a bending towards or from it. This turning 

 phenomenon may now be considered. Before taking up the 

 permanent growth turnings, however, we may consider a case of 

 transitory growth, which throws valuable light upon the true 

 nature of thigmotropism, and serves to connect it with thigmo- 

 taxis. This is the case of the pseudopodia of Orbitolites, 

 which, according to Verworn ('95, p. 429), float at first free 

 in the water after being protruded through holes in the shell; 

 but as soon as they grow longer and heavier they sink in the 

 water, until their distal ends touch the substratum. To this 

 they become attached by a delicate secretion, and grow out 

 along it by the streaming of the protoplasm. The persistent 

 clinging to the substratum is a thigmotropic reaction, and one 

 which belongs clearly to the category of response. 



1. Twining Stems. — The characteristic form of twining 

 plants, like the bean, has long excited the interest of natural- 



