172 



BARDWICKE'S SCIENCE-GOSSIP. 



developed, the appearance, usually, of an oval body 

 consisting of a pair of kidney-shaped cells placed 

 with the concave faces next each other, the space 

 formed in the centre being the pore, or stoma proper. 

 There is a general resemblance between the stomata of 

 all plants however widely separated from each other 

 botanically. In some cases they are longer, and 

 in others nearly circular in outline, and the position 

 of the pair of cells forming the lips, or rim, is 

 generally on a plane, or nearly so, with the surface of 

 the cuticle. 



In order to get a good view of the stomata in the 

 capsules of Orthotrichum, the entire fruit (Dr. 

 Braithwaite says " the lower part," but see further) 

 should be cut longitudinally into two or more parts, 

 in order that they may lie flat under the cover-glass, 

 the spores must be washed away, and the spore sac 



Fig. 96. 



removed ; but this last, is not absolutely necessary, 

 as I was able to find them in specimens already 

 mounted without that precaution having been taken. 

 They occur chiefly on the lower part of the capsule, 

 but are not confined to it. I found some in 

 O. diaphanum and O. affine as high as, and above 

 the middle, and in O. rupestre the highest in one 

 •instance was about one-fifth distant from the top. 



In those species in which the stomata are super- 

 ficial, they are surrounded by the cuticular cells 

 either of the normal shape, or by one or more rows of 

 irregularly shaped smaller ones ; and these differences 

 occur on an individual capsule, so that they form no 

 guide in the determination of species. The general 

 character of the surrounding cells is the same as of 

 those of the rest of the epidermis. (See Fig. 94, 

 O. rupestre. ) 



The species in which the stomata are "immersed" 



present rather more difficulty. The stomatic cells are 

 contained in the wall of the capsule and are 

 precisely of the same form and construction as those 

 which in other cases are exposed, but their outline is 

 completely hidden through being covered over by 

 cells of the epidermis. In some cases a considerable 

 proportion of the inner sides of the stomatic cells is 

 left exposed ; in others only the central pore is 

 exposed ; and sometimes the stoma is completely, 

 apparently hermetically, covered. I found this the 

 case in O. saxatile. 



The cells which surround or cover the stoma are 

 very distinct from those of the rest of the epidermis ; 

 their walls are thicker, smooth, more evenly defined 

 in outline, and semicircular on the sides forming the 



Fig. 97. 



Fig. 98. 



orifice, or pit ; and where the stoma is completely 

 covered the presence of these cells indicates its 

 position. The outline of the stomatic cells can be 

 dimly seen through the tissue by focussing, or may be 

 found by turning the inner wall of the capsule upper- 

 most. 



These thickened cell-walls overlap one another and 

 can'only be seen in their entirety by altering the focus. 



If the section be examined before the removal of 

 the spore sac, when it retains somewhat of its natural 

 form, the cells covering the stoma appear as dark 

 patches, and by the above means are found to form 

 very slight elevations on the surface of the cuticle. 

 The difference between the two,' kinds of cell- 

 walls does not necessarily imply an essential 

 difference in structure, as those in the body 



