106 HARSHBERGER— COMPARATIVE LEAF STRUCTURE [April 24, 



that their long axes are placed in a line with the incident rays of 

 light that strike the upper surface from above and the lower surface 

 by reflection from the sand below. A staurophyll (Fig. 9, 

 Plate II.). 



Sesuvium portnlacastrum. — The leaf structure of this member of 

 the family Aizoace?e is that of a typical diplophyll, but with a slight 

 indication of the staurophyll arrangement of the cells. The stomata 

 present on both sides of the leaf are slightly sunken and the guard 

 cells incline inward and downwards. The upper and lower palisade 

 tissues show four to five layers of cells. The leaves are thick and 

 succulent. A diplophyll (Fig. 10, Plate II.). 



Ipom<ra pcs-caprcc. — This tropical, seaside morning glory is a 

 typical plant of the sandy beaches in Mexico, the West Indies and 

 Bermuda. It grows down off the dune slopes onto the beach sand 

 as a creeping plant, a distance of twenty to thirty feet (Fig. i, Plate 

 I.). The leaves are alternate, elliptical, retuse at the apex and 

 frequently when the sun is hot and the reflection from the sand 

 intense, the leaves fold together along the midrib and stand vertically 

 so as to receive the incident rays of light on the upturned edges of 

 the leaves. The walls of the epidermal cells on both sides of the 

 leaf are thick. The stomata on both sides are sunken about half 

 the thickness of the epidermal cells and the palisade tissue is promi- 

 nent on both sides, constricting the loose parenchyma to a narrow 

 layer. The leaf is, therefore, a true diplophyll (Fig. 11, Plate III.). 



Cakile ccqualis. — This cruciferous plant grows on open, sandy 

 beaches in a more or less scattered manner. It branches in a much 

 more open way than C. maritima, found in similar habitats on the 

 sandy beaches of the eastern United States. The leaves are fleshy 

 and the walls of the upper and lower epidermal cells are thickened. 

 The stomata, which are partly sunken, are found on both the upper 

 and the lower leaf surfaces. The palisade tissue on both sides is 

 five layers of cells thick and the loose parenchyma is restricted to a 

 narrow layer four cells thick in the central part of the leaf. This 

 plant is fitted to its environment by the possession of succulent 

 leaves, epidermal cells with thick walls, and many-layered palisade 

 tissue. A diplophyll (Fig. 12. Plate III.). Contrast the leaf section 

 of Cakile uiantima (Fig. 12 A, Plate III.). 



