170 COLONIAL REPORTS MISCELLANEOUS. 



A taint of the tropopliilous habit is often to be observed in 

 both xero.phytic and hygrophytic districts, but it is not pro- 

 nounced, and is generally restricted to a thinning of the foliage 

 merely, and does not extend to complete defoliation. 



Climatic Formations. 



The differentiation of the earth's vegetation is controlled by 

 three factors heat (determining the zones), atmospheric precipi- 

 tation, and winds (determining districts), and soil ; the latter of 

 which is more local in its influence, and brings about edaphic 

 formations. 



Two fficological groups are to be distinguished ; they are : - 



(1.) Climatic or district formations, governed by the atmo- 

 spheric precipitations. 



(2.) Edaphic or local formations, determined by the nature 

 of the soil. 



The climatic formations can all be traced back to three main 

 types, viz., Woodland, Grassland, and Desert. 



The first two types are closed formations, in which only such 

 plants can thrive as have acquired an optimum adaptation to 

 all the external factors, and this fact enables them to keep out 

 less favoured competitors. Here the struggle is keen, and 

 numerous seedlings perish in consequence ; more components 

 cannot be admitted. Deserts, on the other hand, are open 

 formations, in which the conditions are so severe that only a 

 few plants are able to occupy them, and there is plenty of space 

 available for others if the latter could be found. 



The components of woodland formations consist of woody 

 plants, generally trees. The latter are cosmopolitan in their 

 distribution, and their growth is entirely prevented only by such 

 a degree of drought (due to scarcity of moisture in the soil or to 

 its frozen condition) as excludes all kinds of plants, with the 

 exception of the lower cryptogams. The depth of the root 

 system of trees enables them, to thrive in areas where long seasons 1 

 of drought accompanied by great heat recur periodically. Pro- 

 viding there is a continuous supply of moisture within reach of 

 the root-tips, it is immaterial at what season of the year the 

 supply is renewed. 



Dry winds, however, on account of their pronounced action in 

 increasing transpiration, are decidedly hostile to tree growth, as 

 they tend to reduce the supply of moisture even at great depths 

 of the soil, and their effect is more felt at the greater heights 

 above the surface of the ground, that are usually attained by 

 the tops of trees, than at lower levels closer to the surface. 



Grassland, on the other hand, is mainly occupied by grasses 

 and herbaceous plants; also, though less frequently, by shrubs 

 and dwarfed trees. The latter are, however, very subordinate 

 constituents of the typical formation. Owing to their shallow 

 root systems, grasses are practically dependent on the supply of 

 moisture in the upper superficial layers of the soil, and this 

 supply is continually exposed to the danger of being considerably 



