THE AXGIOSPERMAE : LEAVES 999 



uncommon occurrence and in the Solanaceae, for example, it often gives rise 

 to striking departures from the normal relationships of leaf and shoot. 



Summary of Elementary Anatomy. 



The flat blade or lamina of a dicotyledonous leaf is typically a dorsiventral 

 structure, the upper or adaxial face difl'ering from the lower or abaxial face. 

 Among Monocotyledons on the other hand, dorsiventrality is confined to 

 types with horizontal leaves, those which are borne vertically being often 

 bifacial, i.e., with both sides alike, or they may be round, triangular or 

 square in section, without distinction of upper and lower sides, when they 

 are called unifacial (Fig. loii). 



There is an upper and a lower epidermis, both composed of somewhat 

 flattened cells and covered with a cuticle, which varies greatly in thickness, 

 and may be itself covered by a waxy incrustation. The epidermis of both 

 surfaces frequently bears trichomes in the form of hairs or secretory glands. 

 In the epidermis are the stomata, which are normally confined to the lower 

 surface, but which may be found on both surfaces in a minoritv of Dicotyle- 

 dons and very frequently in Alonocotvledons. 



Each stoma consists of a pair of more or less curved guard cells, whose 

 upper and lower walls are thickened. The guard cells enclose between them 

 a slit-like opening, called the stomatal pore, which communicates with the 

 intercellular spaces of the leaf. The guard cells are distinguished from the 

 other epidermal cells bv possessing chloroplasts. In certain families there are 

 also subsidiary or auxiliary cells, which diff^er from the other epidermal 

 cells by their smaller size and sometimes by their shape. There may be only 

 two of these, one adjacent to each guard cell, or there may be a group of four 

 or more surrounding the stoma (Fig. 985). 



The guard cells show characteristic variations of turgor, often with a day 

 and night rhythm. Increased turgor increases their curvature and this widens 

 the pore between them, and vice versa. When they become flaccid the pore 

 is closed, which usually occurs at night. The variations of turgor may be 

 osmotic, due to increased sugar concentration in their sap, and in this con- 

 nection it should be remembered that the presence of chloroplasts implies 

 the power of photosynthesizing sugars. There is evidence, however, tor 

 another view, that the swelling of the guard cells is due to an increased 

 imbibitional power of their contents, conditioned by a rise of acidity in the 

 sap. The physiology of the movement is not yet fully understood. 



The stomata are exceedingly numerous, 100 to 200 per sq. mm. being 

 not uncommon, but the number is very variable, not only between plants 

 growing under diflFerent conditions, but between younger and older leaves 

 of the same plant, and even in diflFerent parts of the same leaf. The latter 

 variation is probably due to differences in the growth rate of particular areas 

 of the leaf during its development, which we have described in an earlier part 

 of this chapter. In Dicotyledons the stomata are scattered irregularly over 

 the surface, but in Monocotyledons they usually form longitudinal rows. 



