ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 769 



Structure of Leaves-* — E. Mer has studied the cause of the 

 different forms of cells in terrestrial and in aquatic leaves. The 

 structure of a leaf with a well-developed blade and petiole, in which 

 the normal position of the former is horizontal, is due to its situation. 

 The upper face receives a large amount of light, and is in conse- 

 quence well nourished, and the cells of the upper parenchyma acquire 

 a great increase in length, or become palisade- cells. The epidermal 

 cells of the upper surface, well nourished in consequence of their 

 vicinity to the assimilating parenchyma, increase actively and regu- 

 larly, and acquire polyhedral forms, with thick walls and a still 

 thicker cuticle ; the active development of which they are the seat 

 prevents the accumulation in them of food-materials and the forma- 

 tion of stomata. The parenchyma of the lower surface receives less 

 light and is in consequence less well nourished. Its cells grow 

 transversely, and finally separate, leaving between them larger or 

 emaller lacunae ; their walls sometimes become slightly wavy. The 

 cells of the hypodermal layer of the lower surface do not increase in 

 length transversely nearly so much as those of the upper surface, 

 and even become rounded. 



The hairs originate in the bud, and chiefly on the lines of maximum 

 nourishment, the veins. The stomata always make their appearance 

 at the end of the hyponastic or commencement of the epinastic 

 period, during that phase of development included between the com- 

 mencement of the increase in length of the palisade-cells, and the 

 appearance of the waviness in the epidermal cells of the lower surface. 



In the submerged leaves of aquatic plants the sinuosity of the 

 walls of the epidermal cells is due to insufficiency of nutriment. 



Transparent Dots in Leaves.f — P. Blenk has made an ex- 

 haustive examination of the transparent dots in the leaves of a very 

 large number of plants belonging to a great variety of natural orders, 

 from the point of view of their structure and mode of development, 

 and especially of their value in classification. He considers that too 

 little account has hitherto been taken of their presence or absence by 

 Bystematists, the anatomical structure which results in the formation 

 of these dots being often a point of great importance, which may even 

 be made use of in dried specimens. For example, cells with mucila- 

 ginous cell-wall in the interior of the leaf occur only in Anonaceae 

 and Laurineae, and may possibly indicate a close relationship between 

 these orders. 



The various causes of transparent dots or lines in leaves are the 

 following : — Secreting cells, round intercellular secreting spaces of 

 either lysigenous or schizogenous origin, secreting passages, epider- 

 mal or parenchymatous cells with mucilaginous cell-walls, cells con- 

 taining mucilage, raphidcs-cclls, cells with single crystals or clusters 

 of crystals, cystoliths, spicular cells, branched sclcrcnchymatous 

 bundles, groups of sclcrcnchymatous cells, depressed pits with or 



♦ Bull. 8oc. I5ot. France, xxx. (1883) pp. 110-30. 



t Flora, Ixvii. (1884) pp. 49-57, 97-112, 13G-H, 201-10, 223-5, 275-83, 

 291-9, 339-49, 355-70, 371-8G. 



