Fig. 36

Contact with a solid body favours the formation of stolons. Fig. 36 shows a piece of a stem of Pennaria another hydroid, which was lying on the bottom of an aquarium and which formed stolons at both ends a and b. In Margelis, another hydroid, the writer observed that without any opera­tion the apical ends of branches which were in contact with solid bodies continued to grow as stolons, while those surrounded by sea water continued to grow as stems.

Herbst discovered a very interesting form of hetero­morphosis in certain crustaceans; namely, that in the place of an eye which was cut off, an entirely different organ could be formed, namely, an antenna. He showed that the experi­menter has it in his power to determine whether the crustacean shall regenerate an eye or an antenna in place of the eye. The latter will take place when the optic ganglion is removed with the eye, the former when it is not removed. These experi­ments were carried out successfully on Palæmon, Palæmonetes, Sicyonia, Palinurus, and other crustaceans.

The influence of gravitation is very familiar in plants; in stems of Bryophyllum placed hori­zon­tally the roots usually come out from the lower end of the callus. Such phenomena are not often found in animals but they exist here too as the following observa­tion shows.

Fig. 37

Fig. 38

If we cut a piece a b (Fig. 37), from the stem s s of Anten­nu­laria an­ten­nina (Fig. 38), a hy­droid, and put it into the water in a hori­zon­tal pos­i­tion, new stems c d (Fig. 37) may arise on its upper side. The small branches on the under side of the old stem a b begin sud­den­ly to grow ver­ti­cal­ly down­ward.[180] In ap­pear­ance and func­tion these downward-growing elements are entirely different from the branches of the normal Antennularia; they are roots. In order to understand better the trans­forma­tion which thus occurs in these branches, it may be stated that under normal condi­tions they have a limited growth (see Fig. 38), are directed upward, and have polyps on their upper side. The parts which grow down (Fig. 37) have no polyps, but attach themselves like true roots to solid bodies. Thus the changed posi­tion of the stem alone, without any opera­tion, suffices to trans­form the lateral branches, whose growth is limited, into roots with unlimited growth. The lateral branches on the upper side of the stem do not undergo such a trans­forma­tion into roots except in the immediate surroundings of the place where a new stem arises. It seems that the forma­tion of a new stem also causes an excessive growth of roots, possibly because the forma­tion of new branches causes the removal of substances which naturally inhibit the forma­tion of roots. If a piece from the stem be put vertically into the water with top downward, the uppermost point may continue to grow as a stem, while the lowest point may give rise to roots. In this case, therefore, a change in the orienta­tion of organs has the effect of changing the character of organs.