. Experimental morphology. Protoplasm; Growth. 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 gro
. Experimental morphology. Protoplasm; Growth. 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 categor
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