MOSSES 55 



cates the position of the spore-sack, inside which 

 the spores are formed. Its deep colour is generally 

 due to the fact that it is full of spores. In the 

 capsule under consideration the walls are so thin 

 that the form of the spore-sack is clearly dis- 

 tinguishable, even when it is empty. The same 

 thing is also shown in figs. 13 and 15 of the 

 same plate. 



Stomata. Before passing on, I must not omit 

 to mention one feature which is peculiar to the 

 capsules of certain of the mosses. It is of course 

 well known that the microscope reveals, in the 

 delicate skin of ordinary leaves, the existence 

 of numerous tiny holes, through which air, water, 

 and gases are constantly passing in and out of 

 the body of the leaf. These are the pores, or 

 stomata. Now, though nothing of the kind is 

 found in the leaves of mosses, yet in the capsule 

 wall of some species these pores are constantly 

 present, being situated, as a rule, either near to 

 the base or close to the mouth of the capsule. 

 My readers will doubtless be aware that, in 

 ordinary leaves, each of these tiny openings is 

 surrounded by two crescent-shaped cells, known as 

 the " guard-cells," which alter their shape some- 

 what, according to the condition of the atmos- 

 phere, and thus close or open the pore as outside 

 circumstances may require. These guard-cells 

 are also found encircling the pores of a moss 

 capsule, as shown in Plate IV. fig. 23, which 



