CHAP. VII ATMOSPHERIC PRECIPITATIONS 33 



of rain descending during storms, can mechanically damage parts of 

 plants, and especially young delicate parts. The danger of injury arising 

 from falling rain has certainly been greatly over-estimated. According 

 to Wiesner ^ the weight of a drop of artificially produced rain is 62 gramme, 

 but that of the largest rain-drops observed was only -16 gramme. The 

 velocity of descent of rain is small and approximately constant. The 

 maximum kinetic energy of a falling rain-drop is estimated by Wiesner as 

 0004 kilogramme-metre. 



As a means of protection in this regard, the following devices have been supposed 

 to serve : 



1. The leaves of many, especially tropical, plants are directed upwards or 

 downwards, so that the rain strikes them at acute angles and thus acts less violently ' ; 

 in particular, young parts, either individual leaves or whole twigs, are pendulous 

 and do not erect themselves until they have acquired a firmer texture (many tropical 

 plants, Picea, and others). 



2. Foldings, or corrugations of the leaf-blades, may operate in like manner. 



3. Other plants having compound leaves execute paratonic movements when 

 the sky darkens, before the rain itself descends ; consequently the rain impinges 

 upon the leaflets at more acute angles. 



4. Finely compound leaves of many tropical trees expose, as a whole, a less 

 easily assailable blade than do broad and undivided leaves. 



5. The possession by the leaves of most plants of a certain amount of free 

 mobihty, due to their stalks or other causes, is probably the very best defence 

 against the impact of falling rain-drops. Nothing beyond a shaking of the foliage 

 or branches as a direct mechanical effect can be assumed to take place.' 



Hail can be very injurious to plants ; but there can scarcely be said 

 to be adaptations protective against the damages threatened by hail- 

 storms, though the contrary opinion has been expressed. 



Mist (clouds) absorbs light and thus can obstruct the assimilation of 

 carbon dioxide.* It also retards the heating of the soil. Against it 

 there can scarcely be said to exist any protection. Consequently the 

 coasts of Spitzbergen, Greenland, and other northern lands are barren 

 and poor in vegetation when compared with land distant from the coast. 



The vital and morphological significance of water to the plant in other 

 respects can best be dealt with later on, partly in connexion with the 

 different communities, but a few matters may be noted here : 



A moist climate lengthens the life of individuals and of leaves. Aridity, 

 on the contrary, shortens the vegetative period, hastens blooming, the 

 inception of fruits, the maturation of seed ; also brings into existence a 

 marked resting-season, and in steppes and deserts, very numerous annual 

 species. 



The geographical significance of water is still greater than that of heat, 

 because its distribution is still more uneven ; this is true not only in the 

 main, but also and especially in details. Water is of all factors the most 

 pregnant in relation to kind and distribution of plant-communities. 



The relation between rainfall and the amount of water needed by the 

 plant is of great import in regard to differences in the vegetation. Upon 

 this depends the development of equatorial forest-zones, where the rain- 

 fall is very great, of desert-zones near the two tropics when the rainfall 



* Wiesner, 1895. 



- In this and other devices subsequently mentioned, illumination also plays 

 a part, see p. 19. 



' Wiesner, loc. cit. * See p. 16. 



WARMING D 



