Xll INTROiitit,iI'UK, 



base of the leaf and frequently those at the apex also are more or 

 less modified, and the normal type of areolation of any species 

 must be considered that of the median part of the leaf ; and it is 

 safest to take for examination a point about one-third of the 

 length of the leaf from its apex ; the figures of cells in the Plates, 

 marked i c, are all taken at this point. It is however often of 

 importance to examine also the basal cells ; in this and indeed in 

 all cases it is of importance to examine those of fully matured 

 leaves. 



In addition to the form of the cell, it is necessary in certain 

 cases to note also the thickness or otherwise of the cell-wall. It 

 is hardly possible to define a limit at which a cell-wall becomes 

 incrassate, but a consultation of the figures referred to in the 

 Glossary under that term will give a very fair idea of its 

 signification. The cells figured in Tab. III. 3, 8, may be taken as 

 typically thin-walled cells. The presence or absence of papillae 

 on the surface of the cells is another point requiring observation ; 

 this is best done, not by examining single leaves, but by placing 

 a stem or branch under the microscope, so that the papillae may 

 be observed as it were in profile. In a few cases, more especially 

 where they are confined to the front or upper surface, it is 

 necessary to cut a thin transverse section of the leaf (Tab. III. 

 19, 20). This is also desirable in a few other cases, when it is 

 required to observe the structure or form of the nerve or its 

 appendages, as in Campylopus, Dicranum, and Polytrichum 

 (Tab. III. 10, 12, 14, 18). 



When it is desired to observe the basal cells, and especially 

 those of the basal angles or auricles, it is especially necessary to 

 remove the leaves carefully and gently, as otherwise these special 

 cells are liable to be left adhering to the stem. 



Reproductive bodies, termed gemmae, are not unfrequently 

 produced on or among the leaves ; these are of various forms and 

 colours, but are usually green or brown, globular or club-shaped, 

 articulate structures, resembling to the naked eye grains of dust 

 or pollen ; in certain cases their presence is of great assistance in 

 the determination of species. 



The Reproductive Organs. 



The reproductive organs of mosses are usually situated among 

 specialised leaves, forming the so-called flowers. They are of 

 two kinds, the antheridia, or male organs, and the archegonia, or 



