350 Transactions. — Botany. 



corpuscles (chl.) closely crowded in the centre. The last 

 layer of the palisade tissue contains fewer though still many 

 corpuscles, and is intermediate in shape between the cells of 

 the palisade and spongy parenchyma. Next to the layer is 

 the typical spongy tissue (sp.) of the leaf, with fewer chloro- 

 phyll corpuscles (chl.) irregularly arranged in the cells. On 

 the lower surface there is an epidermis (I. ep.) with hairs (h.) 

 and numerous stomata (st.). The cells of the epidermis are 

 smaller than are those of the upper surface, and are protected 

 by a thin cuticle (cu). The hairs consist of several cells, 

 elongated and narrow, and are similar to the hairs of the pit. 

 The epidermis of the pit (p. ep.) is like that of the lower sur- • 

 face, but the cells are slightly larger. Above and around it is 

 the spongy parenchyma of the leaf, with no intervening layer 

 of colourless cells. The stomata of the lower surface have 

 rather small guard-cells (g.), and slightly modified subsidiary 

 cells ($.). 



The structure of the leaf and tissues surrounding the pits 

 is seen to be essentially the same in all these species. In all 

 cases the leaves have a typical dorsiventral structure, with 

 palisade and spongy parenchyma, showing only slight modifi- 

 cations in the shape of their cells and the number and ar- 

 rangement of the chlorophyll corpuscles they contain. The 

 cuticle varies in outline and thickness ; the stomata in the 

 shape of the guard-cells, the subsidiary cells, and the size of 

 the stomatal opening. The absence of a hypodermal storage 

 layer is usually compensated for by an enlarged epidermis ; 

 when present this layer is seen, on staining, to have living 

 protoplasmic contents. Some species have cells which appear 

 to be in course of differentiation into this layer, as the 

 layer (ho.) of figs. 1 and 15. Others, again, seem to have 

 a colourless layer in course of differentiation from the spongy 

 tissue around the pit, as layer (x.) of figs. 14, 15, and 16. 

 There may be one or two of these colourless layers already 

 present. Finally, the hairs of the pits differ in the num- 

 ber and shape of their cells. They are in all cases out- 

 growths of epidermal cells, covered with a thin layer of 

 cuticle. But the essential features are the same in all cases. 



Development of the Pits. 



I examined very young leaves only in the case of Co- 

 yrosma baueri and Goprosma hicida. Fig. 2 shows a trans- 

 verse section of a young pit of the former, fig. 7 of the 

 latter. The pit begins as a shallow indentation, which 

 gradually grows deeper, and finally becomes partially roofed 

 in by the extension of the surrounding tissue of the leaf. 



Fig. 2 (stained with eosine) : The upper (ep.) and lower 

 epidermis (I. ep.) consist of small tightly packed cells, with 



