dominating feature of the plant. For this reason, this leaflet is in- 

 tended to suggest how the foliage leaf may be used as an object for 

 nature study in such a way, that all work done will have a definite pur- 

 pose and an equally definite value. 



Foliage leaves are so variant in general appearance, in position, in 

 size and general outline, that it seems necessary to determine what 

 characters are common to all such organs. The following general 

 characters will be found to apply to all foliage leaves however diverse 

 they may be in appearance. 



1. The foliage leaf is a lateral organ of the stem. It is found upon 



no other part of the plant body. 



2. The foliage leaf is, eharacteristically green, due to the presence of 



chlorophyll which is developed only in the presence of sun- 

 light. 



3. The foliage leaf is an expanded organ, giving the greatest possible 



surfnce exposure to light and atmospheric conditions. Other 

 ]'iiit.s of the plants are mass structures, not surfaces. 

 It is A cry evident from these common characters, that the foliage 

 leaf is an ctrgnn adapted for the light relation. The value of this con- 

 ception of the foliage leaf in nature studies can scarcely be over-esti- 

 mated. Its application readily and clearly explains peculiarities of 

 form, of position, of lobing, and the great mass of adaptations char- 

 acteristic of plants growing under differing conditions. It explains in 

 a general way plant outlines, and will be found to render clear many 

 apparently puzzling conditions. 



Before illustrating the above points specifically, it will be well to 

 consider briefly the work of the leaf. This work may be grouped 

 under four heads. 



1. Transpiration, or the interchanges of moisture between the in- 



terior of the plant and the external air. The result of trans- 

 piration, which is after all apparently little else than evapora- 

 tion, is to aid in the transfer to the leaves of the nutrient 

 water taken from the soil by the roots. 



2. Respiration, otl breathing. Those gaseous interchanges between 



the plant and the air through which oxygen is taken up by 

 the plant and carbon dioxide returned to the air. 



3. Carbon fixation, or those processes through which, under the in- 



fluence of light, carbon dioxide taken from the air is broken 

 do^^■n, the carbon being retained and built into the tissues of 

 the plant, while a portion at least of the oxygen is returned 

 to the air. 



