398 RELATION TO ENVIRONMENT. 



and to other plants in certain regions that the intense light 

 and heat may harm the protoplast, or the chlorophyll, or both. 



The statement that leaves usually face the light at right angles 

 is to be taken as a generalized one. The source of the strongest 

 illumination varies on different days and again at different times 

 of the day. On cloudy days the zenith is the source of strongest 

 illumination. The horizontal position of a leaf, where there are 

 no intercepting lateral or superior objects would receive its 

 strongest light rays perpendicular to its surface. The fact is, 

 however, that leaves on the same stem, because of taller or 

 shorter adjacent stems, are so situated that the rays of greatest 

 illuminating power are directed at some angle between the 

 zenith and horizon. Many leaves, then, which may have their 

 upper sides facing the general source of strongest illumination, 

 no not necessarily face the sun, and they are thus protected 

 from possible injury from intense light and heat because the 

 direct rays of sunlight are for the most part oblique. This 

 does not apply, of course, to those leaves which "follow the 

 sun" during the day. Their specific constitution is such that 

 intense illumination is beneficial. 



The leaf is adjusted as well as may be in different species of 

 varying constitution, and under different conditions, to a certain 

 balance in its relation to the factors concerned. The problem 

 then is to interpret from this point of view the positions and 

 grouping of leaves. Because of the specific constitution of dif- 

 ferent plants, and because of a great variety of conditions in the 

 environment, we see that it is a more or less complex question. 



769. Day and night positions contrasted. In many plants 

 the day and night positions of the leaves are different. At 

 night the leaves assume a position more or less vertical, known 

 as the profile position. This is generally regarded as a pro- 

 tective position, since during the cool of the night the radiation 

 of heat is less than if the leaf were in a vertical position. In 

 many of these plants, however, the leaves in assuming the night 

 position become closely appressed which would also lessen the 

 radiation. This peculiarity of leaves is largely possessed by 



