i6'4 FORMATIONS AND GUILDS [Part II 



To consider grassland, as is frequently done, as the sign of a 'bad 

 climate,' as an evidence of poverty in Nature, as a transition state between 

 forest and desert, is at best comprehensible from a forester's point of 

 view, but is neither scientifically nor practically justifiable. Indeed certain 

 forms of woodland are climatically more accommodating in their demands 

 than is grassland. Victory in the struggle between woodland and grass- 

 land belongs to the one of the two antagonists with which the given 

 climatic conditions best correspond. 



Accurate knowledge of the demands made on the one hand by woody 

 plants, and on the other by grasses, in regard to atmospheric precipita- 

 tion, movements of the air, and heat, will yield us the elements of which 

 a woodland climate and a grassland 'climate are composed. 



ii. WOODLAND CLIMATE. 



Let us first consider the woody plant in its most complete development, 

 as a tree. In the tree the transpiring surface is at a greater distance 

 from the water-supply in the soil than it is in the shrub or herb ; besides 

 this, the strata of air surrounding that transpiring surface have properties 

 different to a certain extent from those nearer the soil ; finally, at least 

 in many cases, the transpiring surface of the tree is larger when compared 

 with the corresponding surface of the ground than it is in the shrub or 

 herb. 



On the other hand, the tree has at its disposal a vast root-system, 

 which is capable of utilizing very deep-lying supplies of water, and upon 

 these it often mainly relies, as its root-tips for the most part lie at 

 a considerable depth below the surface of the ground. 



Our present knowledge of the physiology of trees is in the main 

 derived from the Central European flora, the trees of which all transpire 

 freely although in an unequal degree, and are correspondingly highly 

 water-demanding and hygrophilous during the vegetative season. The 

 most comprehensive and useful investigations regarding the transpiration 

 of the forest trees of Central Europe have been carried out by R. von 

 Hohnel T , from whose works the following data are taken : — 



The author experimented with seedling-trees 5-6 years old and 50-80 cm. high 

 which had been planted in ordinary garden-pots 16 cm. high and each containing 

 3^-5 kilograms of earth. The pots were surrounded by wide air-tight sheet-zinc 

 cases, made so that not only was it possible to water the plants while the pot 

 was kept completely enclosed, but also that the soil did not come into any 

 contact with the zinc. In this way all loss of water from the soil was excluded 

 and a correct determination of the amount of transpiration secured. The 

 experiment was also so arranged that the pots could not be exposed to the 



1 Von Hohnel, op. cit. 



