§ 2] HYDROTROPISM 355 



Among animals the importance of moisture for the growing 

 young is indicated, by the fact that even in species living on 

 the land or in the air the eggs and larvae are frequently con? 

 fined to moist situations, as in pulmonate gasteropods, in many 

 insects, and in reptiles. When this is not the case, the eggs 

 are provided with thick, water-containing envelopes, as in 

 birds, or are placed on succulent leaves, or in special fluid- 

 filled receptacles, as in many insects. Only rarely, as for 

 example in the case of the meal worm, Tenebrio, and the Der- 

 mestidse, are the young found growing in a very dry medium. 

 Doubtless in such cases the amount of water required for 

 growth is less than in the cases where the larvae develop in 

 moist situations. 



To summarize, a minimal quantity of water is essential to 

 germination and growth ; and above this limit growth pro- 

 ceeds more rapidly with the increase in water up to a maxi- 

 mum which varies with the species. 



§ 2. Effect of Waxee on the Direction of Growth 

 — Hydrotropism 



A growing organ, such as a leaf, root, or stolon, is normally 

 in a condition of turgescence, as a consequence of the imbibi- 

 tion of water. So long as the turgidity is equal on the two 

 sides of the organ the latter retains its normal position. If 

 the turgidity is diminished on one side, the organ bends 

 towards that side ; if it is increased on one side, the organ 

 bends from that side. Thus, variations in cell turgidity cause 

 changes in the position of organs. 



This inequality of turgescence on the two sides of an organ 

 may arise in a homogeneous atmosphere ; for certain organs 

 have the capacity in a dry atmosphere of losing water on one 

 side faster than on the other, and in a moist atmosphere of 

 becoming more turgescent on one side than on the other. Con- 

 sequently, the organ assumes a characteristic position accord- 

 ing as the hygroscopic condition of the atmosphere is high or 

 low. Such hygroscopic movements are of wide occurrence 

 among plants, and are often highly adaptive. We see them, 

 for example, in the folding of vegetative parts of the so-called 

 Resurrection Plant of California (Selaginella lepidophylla), by 



