142 



THE LEAVES. 



237. Alternate Leaves, This general term, which commonly suf- 

 fices in descriptive botany, obviously comprises a variety of modes. 

 There is, first, the case to which the name is strictly applicable, 

 namely, where the leaves are alternately disposed on exactly oppo- 

 site sides of the stem (as in Fig. 168) ; the second leaf being thrown 

 to the side farthest away from the first, while the third is equally re- 

 moved from the direction of the second, and is consequently placed 

 directly over the first, the fourth stands over the second, and so on 

 throughout. Such leaves are accordingly distichous or two-ranked. 

 They form two vertical rows : on one side is the series 1, 3, 5, 7, 

 &c. ; on the opposite, the series 2, 4, 6, 8, and so on. This mode 

 occurs in all Grasses, in many other Monocotyledonous plants, and 

 among the Dicotyledonous in the Linden. 

 A second variety of alternate leaves is 



238. The tristichous or three-ranked ar- 

 rangement, which is seen in Sedges (Fig. 

 170) and some other Monocotyledonous 

 plants. Taking any leaf we please to be- 

 gin with, and numbering it 1, we pass round 

 one third of the circumference of the stem 

 as we ascend to leaf No. 2 ; another third 

 of the circumference brings us to No. 3; 

 another brings us round to a line with No. 

 1, exactly over which No. 4 is placed. No. 

 5 is in like manner over No. 2, and so on. 

 They stand, there fore, in three vertical rows, 

 one of which contains the numbers 1, 4, 7, 

 10 ; another, 2, 5, 8, 11 ; the third 3, 6, 9, 

 12, and so on. If we draw a line from the 

 insertion of one leaf to that of the next, and 

 so on to the third, fourth, and the rest in 

 succession, it will be perceived that it winds 

 around the stem spirally as it ascends. In 

 the distichous mode (237), the second leaf 

 is separated from the preceding by half the 

 circumference of the stem ; and, having completed one turn round 

 the stem, the third begins a second turn. In the tristichous, each 



FIG. 170. Piece of a stalk, with the sheathing bases of the leaves, of a Sedge-Grass (Carex 

 Cms-conn), showing the three-ranked arrangement. 171. Diagram of the cross-section of the 

 same, showing two cycles of leaves. 



