PTEBIDOPHYTES. 



113 



walls. The epidermal cells contain numerous chloroplasts, and on the 

 under surface of Jhe leaf breathing pores (stomata, sing, stoma), not 

 unlike those on the capsules of some of the bryophytes. Each breathing 

 pore consists of two special crescent- shaped epidermal cells (guard cells), 

 enclosing a central opening or pore communicating with an air space 

 below. They arise from cells of the 

 young epidermis that divide by a 

 longitudinal wall, that separates in the 

 middle, leaving the space between. 



By holding a leaflet between two 

 pieces of pith, and using a very sharp 

 razor, cross-sections can be made. 

 Such a section is shown in Fig. 68, 

 A. The epidermis (e) bounds the 

 upper and lower surfaces, and if a 

 vein (/.&.) is cut across its structure 

 is found to be like that of the fibro- 

 vascular bundle of the leaf stalk, but 

 much simplified. 



The ground tissue of the leaf is 

 composed of very loose, thin- walled 

 cells, containing numerous chloro- 

 plasts. Between them are large and 

 numerous intercellular spaces, filled 

 with air, and communicating with the 

 breathing pores. These are the prin- 

 cipal assimilating cells of the plant ; 

 i.e. they are principally concerned in 

 the absorption and decomposition of 

 carbonic acid from the atmosphere, 

 and the manufacture of starch. 



The spore cases, or sporangia (Fig. 

 69), are at first little papillae (.4), 

 arising from the epidermal cells, from 

 which they are early cut off by a 

 cross-wall. In the upper cell several walls next arise, forming a short 

 stalk, composed of three rows of cells, and an upper nearly spherical cell 

 the sporangium proper. The latter now divides by four walls (_B, (7, 

 i-iv), into a central tetrahedral cell, and four outer ones. The central 

 cell, whose contents are much denser than the outer ones, divides again 

 by walls parallel to those first formed, so that the young sporangium now 



FIG. 69. A, 

 sporangium 



mother cell of the 

 of the maiden-hair 

 fern, * 300. B, young sporangium, 

 surface view, x 150 : i, from the 

 side; n, from above. C-E, suc- 

 cessive stages in the development 

 of the sporangium seen in optical 

 section, x 150. F, nearly ripe 

 sporangium, x 50: i, from in front ; 

 n, from the side. an. ring, st- 

 point of opening. G, group of four 

 spores, x 150. H a sinle 

 x 300. 



a single spore, 



