44 EDITH SCHWARTZ CLEMENTS 



Senecio taraxacoides (sub-type): staurophyll goo p.; epidermis, 

 upper 50 p., lower 40 /i ; cuticle 4 /t ; chlorenchym (810 p.) 5 irregular 

 rows prolate palisade cells, the upper 2 rows 175 ft each, the lower- 

 most IOO/1. 



GRINDELIA SQUARROSA : staurophyll 375 /i ; epidermis 25 p. ; cuticle 

 5/1; chlorenchym (325/1) prolate palisade cells 30/1, loosely and 

 irregularly arranged; frequent transverse bands or isolated areas 

 of water-storage tissue, of subglobose and polygonal cells. (Plate 

 V, fig. 3.) 



Psoralea lanceolata: water-storage cells large, oblong ones per- 

 pendicular to the surface. 



Helianthus pumilus: leaf 400/1; cells 50-75/1. 



Helianthus petiolaris: leaf 300 /t ; cuticle 3 /t ; cells 50 p.. 



Helianthus scaberrimus (sub-type): leaf 650/1; epidermis 35/1; 

 cuticle 5/1; cells 50-125/1, the lower ones oblobate. (Plate V, 

 fig- 4-) 



Transitions from mesophytes to xerophytes are represented by 

 those of the latter with diphotic leaves. These differ from meso- 

 phytes in closeness of tissues and increased amounts of palisade 

 tissue. The true xerophytes fall into two broad groups according 

 to leaf structure: those consisting entirely of palisade cells, with 

 water-storage tissue where present arranged in transverse bands, 

 the staurophyll type ; and those with hypodermal palisade cells and 

 central region of sponge or water-storage tissue, the diplophyll type. 

 Xerophytes are found in situations characterized by small amounts 

 of soil-water, high light intensities, and low humidities, whether the 

 latter are brought about by high temperatures, as in the foothills, 

 or by altitude, as in the alpine region. Adaptations in the leaf 

 then must be in the direction of protection against harmful trans- 

 piration and over-illumination. Diplophyll and staurophyll types 

 together with thick cuticles and woolly coverings in single instances, 

 are the results. The cuticle, hairs, compactness of tissues and 

 presence of water-storage tissue control transpiration, while abund- 

 ance of chloroplasts, long palisade cells or numerous compact 

 rows and hairs regulate the light. The presence of the isophotic 

 structure in leaves that are not vertically placed, but are normally 

 horizontal, is clearly due to the effect of reflected light and heat, 

 since such leaves are found in situations where such factors are 

 present. Another point of interest brought out by a study of this 



