MORPHOLOGY 



95 



most needed to facilitate the interchange of gases necessitated by the 

 processes of assimilation. In dorsiventral leaves the stomata occur, for 

 the most part, if not exclusively, on the under surface, and average 

 about 100 to the square millimetre, although in some plants their 

 number may reach 700. Leaves which are alike on both sides 

 have their stomata equally distributed on their upper and under 

 surfaces. Floating leaves of aquatic plants have stomata only on the 

 side exposed to the air. In some cases, as in the Oleander (Nerium 

 Oleander), several stomata are situated together in depressions 

 in the under surfaces of the leaves. In the tissue directly under 

 each stoma there is always a large intercellular air-chamber, termed 

 the RESPIRATORY CAVITY (Fig. 103, B, a), which is in direct com- 

 munication with other intercellular spaces extending throughout the 

 leaf tissue. In plants grown in abundance of moisture, these inter- 

 cellular spaces in leaves are larger than in the case of plants growing 

 in drier situations. 



In contrast to the stomata, which as air-pores serve for the inter- 

 change of gases, a few plants also possess water-stomata or water- 

 pores, situated at the ends of the so-called veins or nerves of the 

 leaves. These pores serve as organs for the discharge of water or 

 watery solutions. Calcium carbonate, in solution, is frequently excreted 

 in this way, and in many species of Saxifraga it forms white scales 

 on the margins of the leaves. Although water-pores may often be 

 found at the apices and tips of the marginal teeth of young leaves, 

 they seem to dry up as the leaves become more mature. The guard- 

 cells of water-stomata always 

 lose their living contents pre- 

 maturely, and thus the passage 

 between them remains con- 

 tinually open. The water- 

 stomata (Fig. 106) are always 

 larger than the air-stomata. 

 Although submerged leaves of 

 aquatic plants are devoid of 

 air - stomata, water - stomata 

 often occur on them. 



Hairs or trichomes and 

 tegumentary outgrowths or 

 EMERGENCES are characteristic 

 of the tegumentary system. 



The simplest form of hairs are Fig. 10e.— Water-pore from the margin of a leaf of 

 the PAPILLAE, which are merely Tropwolum majus, with surrounding epidermal 



epidermal cells, the external cells - (x240 - ) 



walls of which have protruded in a conical form. Papillae are often 

 developed on the petals of flowers, and are the cause of their 

 velvety appearance (Fig. 107). Longer hairs, such as the root-hairs 



