104 HARSHBERGER— COMPARATIVE LEAF STRUCTURE [April 24, 



The taproot is large and strong and from it numerous branches, six 

 to eight inches long, are formed. The leaves are opposite, small, 

 ovate, with an acute apex and barely petiolate. The upper epidermis 

 consists of two rows of cells, the palisade is a single layer and the 

 loose parenchyma is compact. The lower epidermal cells are papil- 

 late and latex is present. The adaptation to the environment is 

 shown in the latex, the two-layered upper epidermis and the over- 

 lapping arrangement of the leaves. A diphotophyll (Fig. 3, 

 Plate 11.) . 



Dodoncea viscosa. — This small sapindaceous tree occurs on the 

 inner edges of the sand dunes. Its leaves are alternate, spatulate 

 with the base narrowed to the point of attachment. The leaves are 

 varnished. The upper epidermal cells are thick and provided with 

 peltate hairs. The palisade cells are disposed in two layers. The 

 loose parenchyma is open, while next to the lower epidermis there is 

 a row of small cells which may be considered as a lower palisade 

 layer. Hence the leaf is a potential diplophyll. The stomata of the 

 upper side are slightly raised above the surface, while those on the 

 under side have developed a small projecting beak (Fig. 4, Plate II.). 



Lantana involucrata is one of the plants that enters the formation 

 of the Bermuda scrub. It also invades the dunes. The leaves are 

 hairy on both surfaces. A section of a leaf shows that the upper 

 epidermis is without stomata, but is provided with straight, multi- 

 cellular and capitate, unicellular hairs. The lower surface shows 

 depressions provided with the capitate hairs, while the raised por- 

 tions of the leaf surface between the depressions is covered with 

 both straight, muticellular and capitate, unicellular hairs. The pali- 

 sade is a single layer. The stomata project outward beyond the 

 general surface of the lower epidermis, but they always occur in 

 the depressions. The depressions provided with hairs and stomata 

 and thick, hairy upper epidermal surface are structures which fit the 

 plant to exist on the hot, sun-exposed sand dunes of the islands. A 

 diphotophyll (Fig. 5, Plate II.). 



Neriiim oleander. — The leaf structure of the oleander, a native 

 of the Mediterranean flora, is well known. The upper epidermis is 

 in three layers with thick cuticle, the paliside tissue in two layers, 

 while the under surface of the leaf is pitted, the pits being filled 



