120 DYNAMICS OF LIVING MATTER 



plish color. The walls are lined with a layer of colorless circulating 

 protoplasm. If, however, the cells are investigated after a prolonged 

 stimulation of the glands of the plant, the basal cells of the tentacle 

 no longer contain a homogeneous liquid, but solid masses of various 

 shape which have the purplish color and are surrounded with an almost 

 colorless liquid. These changes do not necessarily depend upon the 

 bending of the tentacles, but only upon the stimulation of the glands. 

 This process of aggregation (which may be a gelation) is reversible, 

 and after a period of rest the original appearance of the protoplasm 

 is reestablished. By way of digression we may mention that Darwin 

 observed that the process of aggregation traveled from the stimulated 

 gland to the contracting tentacle, and that what he observed here directly 

 may occur invisibly in the stimulated nerve fiber. It is possible that 

 in the positively heliotropic organs a process of aggregation occurs in 

 the cell on the side of the light, while in negatively heliotropic organs 

 the reverse occurs on the light side; and this may be the explanation 

 of Wortmann's observations. On the side where the protoplasm be- 

 comes denser (or undergoes aggregation?) the cellulose walls become 

 subsequently thicker than on the opposite side. 



The same phenomena of heliotropism which we find in plants we 

 find also in sessile animals ; and the identity of the heliotropic reactions 

 in these two groups of organisms is so complete that it would be at 

 any time possible to demonstrate the phenomena and laws of plant 

 heliotropism in such animals, and vice versa. One of the best animal 

 forms in which to show this identity is Eudendrium, a hydroid. As 

 stated in the preceding lecture, the polyps of this hydroid soon fall off 

 when it is brought from the ocean into the aquarium ; but in a few days 

 new polyps are formed, and as soon as this occurs the little stems in 

 the region below the polyp bend toward the source of light, when illumi- 

 nated from one side only (see Figs. 20 and 21). The region in which 

 this curvature occurs is situated immediately below the polyps, and it 

 happens that in this region also the main growth of the stem occurs. 

 The bending of the polyp or the tip of a branch continues until the 

 symmetrical points of the stem are struck by light at the same angle. 

 If there is only one source of light this occurs when the axis of symmetry 

 falls into the direction of the rays of light. As soon as this happens 

 the stem continues to grow in the direction of the rays of light. 



In Eudendrium just as in plants the more refractive blue rays are 

 more effective than the red rays; behind a red screen' the heliotropic 

 curvatures in Eudendrium do not occur at all, or only slowly, while be- 

 hind a blue screen they occur as rapidly as in mixed daylight. 



In Eudendrium we are able to convince ourselves that the region 



