MOSSES AND LIVERWORTS. 107 



II. Cut thin transverse sections of the stems of two 

 or three different species, and study them under the com- 

 pound microscope. Beginning with the outside, notice 



1. The epidermis, consisting of a single layer of periph- 



eral cells. Underneath this, in some of the 

 species, are similar, thick-walled cells, the whole 

 forming a cylindrical band of mechanical tissue. 



2. The cortex, consisting of rounded cells, often con- 



taining starch and oil. 



3. The axial cylinder, an extremely simple form of fibro- 



vascular bundle, occupying the center of the stem, 

 and made up of much narrower elements than 

 those composing the cortex. Longitudinal sec- 

 tions show that these are also much more elongated 

 than the cortical cells are. Observe also whether 

 they differ from the latter in the color of their 

 walls and the character of their contents. 



Leaf. 



I. Examine next the ordinary foliage leaves of the 

 different species, observing 



1. Their differences of size, form, and other external 



features. 



2. Their relation to the stem. Are they stalked or 



sessile? Is their arrangement on the stem alike 

 in the different species ? 



3. The structure of an individual leaf, as far as this 



can be observed under a good lens. Notice par- 

 ticularly the margins and midrib. 



II. Study fresh and well-developed leaves, such as those 

 of new shoots of Mnium, with the compound microscope. 

 The cellular structure will be found beautifully distinct, 



