260 



PRACTICAL BOTAXY 



radially about the stem much more chlorophyll is exposed than 

 could be exposed in the same space by a prostrate plant. The 



importance of the stem in holding 

 these leaves up into the air, thus 

 making the radial arrangement 

 possible, is evident. It is also 

 possible that no less importance 

 should be attached to transpor- 

 tation of water through the stem 

 to the leaves, though it is not 

 known to what extent moss leaves 

 get their water directly through 

 their surfaces or through the stem. 

 The expanded portions of the 

 leaves are a single layer of cells in 

 thickness, while the median por- 

 tion may consist of several layers 

 of cells. In the middle (midrib^ 

 there are vows of elongated cells 

 running from base to tip of the 

 leaf. These constitute the vein of 

 the leaf. 



244. The moss plant : sexual 

 reproduction. The sex organs are 

 borne upon the upper end of the 

 stem. If the terminal leaves are 

 carefully removed from plants 

 that are in reproductiN'e condition, 

 two kinds of sex organs together 

 with some sterile filaments (^para- 

 pht/ses) may be found. In some 

 kinds of mosses but one kind of sex 

 organ grows upon a single plant, 

 \\'hile in otlier kinds both may grow 

 upon the same plant. Magnifica- 

 tion is needed in studying them. 



.Fig. 217. Archegonia of a moss 



At A is shown the tip of a stem of 

 a, leafy moss plant, with the frag- 

 ments of leaves (l) surrounding a 

 group of archegonia (a). At B is 

 an enlarged archegoninm, in tlie 

 swollen part of which is the c^g 

 (e), above it the neck ()(,), contain- 

 ing the neck canal cells, and at tlie 

 end of the neck are tlie cells (in) 

 which later open tfi form the place 

 of entrance for sperms. A, magni- 

 fied 100 times; B, magnified 500 

 times. After Sachs 



