STOMATA AMONG ARCHEGONIATAE 469 



the dorsal walls of the guard-cells are much thicker than the ventral 

 walls, both as a rule being lignified and partially cutinised ; a limited 

 area of the dorsal wall always remains unmodified to permit of the 

 inward and outward movements of water that accompany the turgor 

 variations of the guard-cells. The cavities of the guard-cells are gene- 

 rally narrow, and have their long axes directed obliquely downwards 

 and backwards {i.e. away from the pore). Freedom of movement of 

 the stoma is always ensured by the development of hinge-areas. The 

 stomata of Gymhosperms are almost invariably more or less deeply 

 sunk. According to Copeland, the stomata of Ginkgo and Larix 

 resemble those of Grasses in structure. 



Among Pteridophytes the stomata display a considerable diversity 

 of structure. Those of Equisetum approximate to the ordinary Gymno- 

 spermous type; the dorsal walls of the guard-cells are strengthened by 

 transverse thickening ridges which project into the cavities of the 

 subsidiary cells. Many hygrophilous Ferns have stomata of the 

 "aquatic" type (Fig. 183 c). The guard-cells of Salvinia natans are 

 thin-walled on all sides, and the pore is of the same width throughout. 

 The guard-cells of the closely allied Azolla caroliniana are remarkable 

 in more respects than one. They have an irregularly three- to five-sided 

 outline; the pore is elongated at right angles to the plane of the septa 

 between the two guard-cells, and these septa finally become partially 

 or entirely obliterated. As in Salvinia, the guard-cell walls are en- 

 tirely unthickened. In these two genera, the numerous hairs on the 

 leaves afford sufficient protection against the risk of capillary occlu- 

 sion of the stomata; that, in the author's opinion, is reason why these 

 stomata are not constructed according to the usual aquatic type. 



In the Mosses, 2211 stomata are confined to the sporogonium [sporo- 

 phyte], where they often occur in large numbers on the apophysis, 

 which is the principal organ of photosynthesis and to a smaller 

 extent on the capsule wall [where the latter is green]. The mechanism 

 of the Mnium stoma has already been explained (cf. above, p. 452). 

 A fully developed stoma of Funaria hygrometrica or of Physcomitrium 

 pyriforme consists not of a pair of guard-cells, but of a single annular 

 cell, which is provided with two symmetrically arranged nuclei and 

 owes its origin to the complete coalescence of two primarily distinct 

 guard-cells. 



A single capsule of Polytrichum may bear three different types of 

 stomata, namely: (1) "two-celled" stomata, with the normal paired 

 guard-cells; (2) "one-celled" stomata, with fused guard-cells; and (3) 

 four-celled stomata, in which each of the two primary guard -cells 

 is divided in two by a median transverse wall. 



The meaning of the fusion of the guard-cells which takes place in 



