SECT. I 



MORPHOLOGY 



115 



or 



of moisture, these intercellular spaces are usually larger than in the 

 case of plants growing in drier situations. 



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

 interchange of gases, a few plants also possess water-stomata 

 WATER-PORES, situated at the 

 ends of the veins or nerves 

 of the leaves. These pores 

 serve as organs for the dis- 

 charge of water or watery 

 solutions. Calcium carbonate, 

 in solution, is frequently ex- 

 creted in this Avay, 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 



Fig. 123.— Water-pore from the margin of a leaf of 

 Tropaenlum majus, with sunounding epidermal 

 (X 240.) 



cells. 



usually lose their living con- 

 tents early, and thus the pas- 

 sage between them remains continually open. The water-stomata (Fig. 

 123) 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 emergences are characteristic of the 

 tegumentary system. The cells of the hairs may retain their living 



contents, or die and become filled 

 with air ; in the latter case the 

 hairs appear white. The simplest 

 form of hairs are the papillae, 

 which are merely epidermal cells, 

 the external Avails of which have 

 protruded in a conical form. 



Fro. 124.— Surface of the upper epidermis of a petal P^ipiHae are ofteu developed On 



of Viola tricolor, showing ridge-like projections the petals of flowers, and are the 



on the lateral walls, and protruding papillae. „„„ „ f j-i, • i - 



(X 250.) cause or their velvety appearance 



(Fig. 124). Longer hairs, such 

 as the root hairs (Fig. 173 ?), are also prolongations of single 

 epidermal cells; these are characteristic of definite regions of the 

 root (p. 47), and only a limited portion of the outer wall of the 

 epidermal cell protrudes as a hair. The woolly hairs found in 

 young buds are generally similarly prolonged epidermal cells Avhich, 

 as a protective covering, surround the young growing tissues and 



