RELATION OF LEAF STRUCTURE TO PHYSICAL FACTORS 

 LIGHT, WATER AND HUMIDITY INCREASED 



The only additional point brought out by this small group of 

 plants is the transformation of prolate palisade cells into oblobate 

 cells. These doubtless serve two purposes, that of palisade proper 

 in response to the strong illumination, and that of sponge since the 

 lobes furnish an increase of transpiring surface called forth by the 

 abundant water supply. Chamaenerium has been slightly dwarfed 

 by the excess of the latter. 



VII. SUMMARY 



The spongophyll type of leaf is characteristic of extreme hydro- 

 phytic situations. It is a practically homogeneous structure of 

 simple globose cells enclosing air-passages, and is in accord with the 

 surrounding medium of water and diffuse light. The spongophyll 

 is also characteristic of monocotyledons where it is hereditary rather 

 than adaptational. At the other extreme as regards both physical 

 conditions and leaf structure is found the staurophyll as character- 

 istic of intense xerophytes. It is composed entirely of prolate cells 

 usually to the exclusion of any considerable amount of air space, 

 or it is combined with more frequent intercellular spaces and water- 

 storage cells. The diplophyll is closely connected with the stauro- 

 phyll and the two comprise the type of leaf called " isolateral " by 

 Heinricher and studied by him in connection with the vertical posi- 

 tion of leaves. He has ascribed to light by far the greater influence 

 in producing this structure, and has given to dryness a very sub- 

 ordinate value. It is true that light is the important factor, but 

 for the extreme development of the type, lack of moisture must 

 play an active part. This is evident when it is remembered that 

 besides being the form best adapted to great assimilative activity and 

 to the prevention of over-illumination, the prolate form of cell per- 



