LEAF REDUCTION 187 



period of vegetation may be a relic of this habit v/hich, in 

 our latitude, fits in with the necessity of obtaining a supply 

 of light before the woods, where for example the wild 

 hyacinth lives, are in full foliage. 



Trees with leaf-fall in summer are characteristic of many 

 tropical and sub-tropical forests. Of the four types of 

 tropical forest distinguished by Schimper two, the monsoon 

 forest and the savannah forest, are more or less leafless in 

 the dry season. The teak forests of East Java are an 

 example of the former, and in the season from June to 

 October the trees are completely bare. In the savannah 

 forests of Venezuela and Brazil the dominant leguminous 

 trees, especially species of Cassia, shed their leaves in the 

 dry season ; there also occur evergreen trees with hard 

 leathery leaves. The " caatinga " of Brazil and the thorn 

 scrub of Mexico are very open communities of a bushland 

 type in which leguminous shrubs, shedding their leaves 

 in the dry season, are prominent. Similar conditions 

 prevail in the woods of the Abyssinian highlands where the 

 dominant Boswellia casts its leaves in drought. 



Leaf Movement and Profile Position. — Temporary reduc- 

 tion of leaf surface of another kind is shown by those plants 

 with pulvinate leaves which assume a definite sun position. 

 We have already mentioned the vertical position of the 

 leaflets of Robinia and of Oxalis in the sun, and how this 

 position avoids extreme insolation and overheating. The 

 most important consequence is probably a reduction in 

 transpiration rate, but the relation is not simple, for the 

 stomata of Robinia are confined to the lower surfaces of 

 the leaves and thus become exposed. V/e might also 

 regard the leaves which take up a permanent profile position 

 from this point of view. The actual transpiring surface is 

 not reduced, but the heat-absorbing surface is, and this 

 is a very important point. 



Types of Small Leaves.— Leaf surface is permanently 

 reduced in many characteristic xerophytes, and a number 

 of distinctive leaf types may be recognised. The pines 

 have characteristic thick narrow leaves which stand out so 



