LIVERWORTS A. YD MOSSES. 



57 



64. The leaves are also more highly developed than in 

 liverworts. They are always sessile and are arranged in two 

 (rarely), three, or more vertical ranks along the stem, and 

 consist usually of a single sheet of chlorophyll-bearing cells, 

 the blade (figs. 69, 70), and a central rib running from base 

 to apex (frequently wanting), which is composed of elongated 

 conducting and strengthening cells (figs. 69, 70). In some 

 the amount of green tissue is increased by the formation of 

 vertical plates similar to the blade (fig. 70). 



65. Branching. — The stem branches, often very profusely, 

 by the formation of lateral growing 

 points beneath the developing leaves. 

 Sometimes the growth of the lateral 

 branches, as of the original main 

 axis, is checked by the formation 

 of sexual organs. In that case a 

 new branch is likely to arise some 

 distance below the apex, so that the 

 stem is a succession of lateral 

 branches, called a sympodium (fig. 

 71). This mode of branching is 

 termed sympodial. In other cases 

 the main axis continues its growth 

 unchecked, and more or fewer 

 branches also develop. These lie 

 plainly upon the sides of a central 

 axis. This mode of branching is Fl 

 called monopodial. Often the 

 growth of the lateral axes is defi- 

 nitely limited and their develop- 

 ment regular, forming a pinnate 

 branch-system. If the secondary 

 axes themselves branch, there is formed a bipinnate or 

 even tripinnate system, as in figure 67, B. 



'ig. 71. — Axis of a moss (Ortho- 

 trichum) showing sympodial 

 branching. S 1 , 6" 2 , S 3 , S 4 , succes- 

 sive clusters of sexual organs, 

 produced at apex which check 

 the growth of axis. Beneath 

 each a lateral growing point 

 develops, produciug successively 

 the brances 6 l , b' 1 ^ b 3 . Magni- 

 fied 10 diam. — After Bruch & 

 Schimper. 



