64 



THE CELL 



V 



\ 



although it does not follow with certainty from the foregoing that it has 

 contributed also to the formation of living substance. 1 



With regard to the original formation of cells and tissues., there are 

 numerous data which go to show that the most widely different stimuli can 

 effect an essential modification of the growth process. 



Here belong the effects of gravitation (geotropism), and other mechanical 

 agencies (rlieotropism, thigmotropism) , of light (helio- 

 tropism}, and of galvanism (galvanotropism) upon the 

 orientation of plants and of various sessile animals. 2 



The following may be mentioned as examples. The 

 stems of plants grow away from the center of the earth 

 (negative geotropism), the roots toward the center of the 

 earth (positive geotropism). If germinating seeds be placed 

 on a wheel rotating rapidly in a vertical plane so that the 

 influence of gravitation is overcome, the stem grows toward 

 the middle point of the wheel, the roots turn away from it 

 i. e., the stem grows in the direction of least pressure, the 

 root in the direction of the greatest (Knight). 



The hydroid polyp (Antennularia antennina) consists of 

 a central stem of 12 mm. thickness, and often more than 

 20 cm. in length, which generally grows perfectly straight 

 up from a tangle of very thin filamentous rootlets. If now 

 an Antennularia whose stem is in process of growth be 

 brought into a position deviating from the vertical, the 

 growing tip bends until it finds the vertical direction again, 

 and then grows directly upward. The root on the other 

 hand grows vertically downward but not in so straight a 

 line as the stem (cf. Fig. 37). 



Rheotropism. Seedlings of maize and other plants ger- 

 minated in a tub of flowing water grow with roots paral- 

 lel to the surface of the water and against the current. 



The hydroid polyp, Eudendrium, also bends in its growth against the current 



(Loeb). 



Thigmotropism. Numerous plants twine around the vertical stems of other 



plants and so climb upward. 



Heliotropism. The growing parts of a plant always turn toward the light. 



FIG. 37. A shoot of 

 Antennularia an- 

 tennina, a small 

 hydroid animal, 

 exhibiting negative 

 geotropism, after 

 Loeb. 



1 Perhaps a clearer case of the influence of nervous tissue on the formation of living 

 substance is that of the regeneration of a " head " in a simple worm. C. M. Child has 

 shown that if the anterior end of the flatworm Leptoplana be cut off in such a way as to 

 leave the collection of ganglion cells which serves the animal as a " brain," the animal will 

 regenerate a new " head "; but if the cephalic ganglia be removed with the anterior end no 

 " head " is regenerated, because in this instance the anterior end is no longer capable of 

 functioning as a " head." In other words, the determining factor in the formation of the 

 living substance here, as in the mammalian muscle, is the motor activity dependent upon 

 the nervous system, or, as we have just learned, a special kind of chemical stimulus. ED. 



2 With Herbst I employ the term geotaxis, galvanotaxis, etc., for the effects on the 

 movements of free-living organisms brought about by external stimuli, and the terms 

 geotropism, galvanotropism, etc., for the changes in growth brought about by external 

 stimuli. The former phenomena are purely dissimilative, the latter are essentially assim- 

 ilative. These phenomena could only be produced by the constant effect of these stimu- 

 lating agents acting in a perfectly definite manner. 



