THE GREENWOOD TREE. 153 



forming a great dome-shaped or umbrella-shaped mass, 

 every part of which stands an even chance of catching 

 its fair share of carbonic acid and solar energy. 



The shapes of the leaves themselves are also largely 

 due to the same cause, every leaf being so designed in 

 form and outline as to interfere as little as possible with 

 the other leaves on the same stem, as regards supply 

 both of light and of carbonaceous foodstuffs. It is only 

 in rare cases, like that of the water-lily, that perfectly 

 round leaves occur, because the conditions are seldom equal 

 all round, and the incidence of light and the supply of carbon 

 are seldom unlimited. But wherever leaves rise free and 

 solitary into the air, without mutual interference, they 

 are always circular, as may be well seen in the common 

 nasturtium and the English pennywort. On the other 

 hand, among dense hedgerows and thickets, where the 

 silent, invisible struggle for life is fierce indeed, and 

 where sunlight and carbonic acid are intercepted by a 

 thousand competing mouths and arms, the prevailing 

 types of leaf are extremely cut up and minutely sub- 

 divided into small lace-like fragments. The plant in such 

 cases can't afford material to fill up the interstices 

 between the veins and ribs which determine its under- 

 lying architectural structure. Often indeed species 

 which grow under these hard conditions produce leaves 

 which are, as it were, but skeleton representatives of 

 their large and well filled-out compeers in the open 

 meadows. 



It is only by bearing vividly in mind this ceaseless 

 and noiseless struggle between plants for their gaseous 

 food and the sunshine which enables them to digest it 



