Mateeqal for Illustration : 



Ivy, geranium, star cucumber, begonia, common maUotv, rag-weed or any 

 plant with lobed or dissected leaves. 



It will be seen then that leaf form largely determines the outline- 

 of the plant taken as a whole. Let us return to the mullein for a mo- 

 ment. It will be remembered that the leaves are entire, the lower ones 

 being the largest and standing nearly at right angles to the stem. As 

 the summit is approached the leaves become gradually smaller and at 

 the same time more closely appressed to the stem, until at the extreme 

 summit they are much reduced and nearly parallel to the stem. This 

 arrangement, so evidently for the purpose of preventing shading of 

 lower leaves, serves to give to the whole plant a general pyramidal 

 outline, a form characteristic of simple plants with entire leaves. In 

 the case of the rag- weed, on the other hand, since there is no diminu- 

 tion in size of the upper leaves, the general outline of the plant is 

 cylindrieal, a form characteristic of plants with divided or dissected 

 leaves. 



It is evident that in genuine nature work the foliage leaf is to be 

 studied from a new view point. It is not to be used as a frame upon 

 which to hang definitions as to form and margin, apex and blade, but 

 is to be considered as a working organ charged with important duties 

 which can only be successfully performed in the presence of the light. 

 In this view all peculiarities of position and form and structure are 

 but devices for enabling the leaf to properly accomplish its work. The 

 main question in every case concerning the foliage leaf is, "How is the 

 light relation secured?" 



Before considering specifically how the view of the foliage leaf as 

 the organ of light relation serves to explain many so-called adaptations 

 to meet special conditions, it is necessary to touch very briefly upon 

 the relation of plants to the soil. It is evident that by far the greater 

 part of the food of the plant is derived from the soil. It is also plain 

 from our knowledge of the structure of the plant, that this food must 

 be taken up in the form of a watery solution. It follows, therefore, 

 that the amount of water in the soil has a very important bearing upon 

 the food supply of the plant, and serves, perhaps, more than any other 

 one factor to determine its structural features. Indeed this matter of the 

 available water of the soil is of such great import that it determines 

 largely not merely the external form of the plant, but also modifies 

 in a marked way its minute structure. 



Based upon this dependence of plants upon and their relation to 

 water, the plants of any given region may be separated into three 



