THE BRYOLOGIST. 



Vol. V. July, 1902. No. 4. 



THE PERISTOHE. II. 



By a. J. Grout. 



In the article in the April, 1901, Bryologist the structure of the peri- 

 stome in Georgia was discussed and some mention was made of its function. 

 Since writing that article the hygroscopic activities of various types of j^eri- 

 stome have been investigated with considerable care, necessitating a few 

 further statements with reference to Georgia. 



It has long been recognized that moss peristomes are strongly hygro- 

 scopic, i. e , respond by active motions to any changes in the amount of 

 moisture in their tissues. It has also been recognized in a general way that 

 the peristome played some part in the distribution of the spores and that its 

 hygroscopic activity aids in this work, but very little attention seems to have 

 been paid to the details or to the extreme nicety with which the peristome in 

 different species has been adapted to do its work. 



The spores of mosses must depend upon currents of air for distribution, 

 hence they must be securely protected from rain or dew, which would mass 

 and clot them together so that they would fall directly to the ground as 

 soon as liberated, to say nothing of the danger of premature germination and 

 decay. 



Then, again, the spores must be liberated in small quantities so that 

 they will not all be discharged at once, but take advantage of breezes from 

 different directions and be sown at various seasons. They must also be well 

 separated or sifted so as to be as widely separated as possible when they 

 finally alight. This sifting of the spores is accomplished by various inter- 

 esting devices which are specially prominent in mosses with pendent or 

 horizontal capsules. In mosses with a double peristome the inner peristome 

 is usually the sieve while the outer protects from water by closing hygro- 

 scopically in wet weather. In mosses with a single peristome both functions 

 are often performed in a very interesting manner by the single row of teeth. 



In mosses with upright capsules there is less need of a so finely meshed 

 sieve, as the spores will not fall out but will be shaken out after the manner of 

 lily seeds. To assist in this shaking the seta is often almost as elastic when 

 dry as a steel wire and if bent to one side flies back when released with a 

 jerk which scatters a small cloud of spores. In wet weather not only do the 

 peristomes close, much after the manner of Chickweed pods, but the seta be- 

 come soft and flaccid. As the highest development of this sifting arrange- 

 ment is of no special advantage to mosses with an erect capsule, the 

 inner peristome has become more or less vestigial in those mosses which 

 have erect capsules, although they maybe most closely related to species 

 having cernuous or pendent capsules with a highly developed inner peri- 



The May Bryologist was issued May ist, iqo2. 



