236 



BLBMBNTS OF BOTANY. 



ap^ 



,rh 



Fig. 202. — A Plant of 

 Pigeon-Wlieat Moss 

 {JPolytrichum commune). 



rhf root-like portion ; a, 

 bristle-like stalk of urn, 

 or spore-case ; c, hoodlike 

 cover of urn ; ap, knob at 

 base of urn ; d, cover of 

 urn. (Natural size.) 



In some specimens the stem may be found, at 

 a height of an inch or more above the roots, to 

 bear a conical, basket-shaped enlargement, out 

 of the centre of which a younger portion of the 

 stem seems to proceed, and this younger por- 

 tion may in turn end in a similar enlargement, 

 from which a still younger part proceeds. 



Note the difference in general appearance 

 between the leaves of those plants which have 

 just been removed from the moist collecting- 

 box and those which have beeh lying for half an 

 hour on the table. Study the leaves in both 

 cases with the magnifying glass in order to find 

 out what has happened to them. Of what use 

 to the plant is this change ? Put some of the 

 partially dried leaves in water, in a cell on a 

 microscope slide, cover, place under the lowest 

 power of the microscope, and examine at inter- 

 vals of ten or fifteen minutes. Finally sketch 

 a single leaf. 



293. Minute Structure of the Leaf and Stem. 

 — The cellular structure of the pigeon-wheat 

 moss is not nearly as simple and convenient 

 for microscopical study as is that of the smaller 

 mosses, many of which have leaves composed, 

 over a large part of their surfaces, of but a 

 single layer of cells, as shown in Fig. 205. If 

 any detailed study of the structure of a moss 

 is to be made it will, therefore, be better for 

 the student to provide himself with specimens 

 of almost any of the smaller genera, i and work 

 out what he can in regard to their minute 

 anatomy. 



294. Spore-Capsules. — That part of the 

 reproductive apparatus of a common moss 

 which is most apparent at a glance is the urn 

 or spore-capsule, Fig. 202. This is covered until 

 it reaches maturity with a hood which is easily 

 detached. Remove the hood from one of the 



* As Mnium or Bryum. 



