138 



ORGANOGRAPHY. 



Fig. 264. 



Tig. 264. Portion of a branch of 

 the Lime-tree, with four leaves 



gence or distance between two successive leaves is \ the circum- 

 ference of a circle, 360° -7-^ = 180°. This arrangement is the 

 normal one in all Grrasses, and many other Monocotyledonous 

 Plants ; and the Lime-tree, and other 

 Dicotyledonous Plants, exhibit a simi- 

 lar arrangement. 



A third variety of arrangement in 

 alternate leaves is the tristichous or 

 three-ranked {fig. 265). Thus, if we 

 start with any leaf, and mark it No. 

 1, and then pass to 2, 3 and 4, we 

 shall find that we shall make one 

 turn round the stem, and that the 

 fourth leaf is vertically over the first, 

 and thus completes a cycle composed 

 of three leaves. In like manner, the 

 fifth leaf will be over the second, the 

 sixth over the third, and the seventh 

 over the fourth and first, thus com- 

 pleting a second cycle; and so on 

 two-rfnked manne"''^°"^ ""^ ^^^^ ^^^® succeeding leaves. Here the 

 angular divergence is |, or one turn, 

 and three leaves, that is360°-=-i = 120. This arrangement is by 

 far the more common one among Monocoty- 

 ledonous Plants, and may be considered as the 

 most characteristic of that class of plants, just 

 as the pentastichous arrangement is of Dicoty- 

 ledons. 



A fourth variety of Phyllotaxis in alternate 

 leaves is the octastichous or eight-ratikcd. Ex- 

 amples of this variety occur in the Holly and 

 Aconite. In this the ninth leaf is over the 

 first, the tenth over the second, the eleventh 

 over the third, and so on ; thus taking eight 

 leaves to complete the cycle ; and, as the 

 spiral line here makes three turns round the 

 stem, the angular divergence will be | of the 

 circumference, 360°-r| = 135°. 



The above are the more common varieties of 

 Phyllotaxy ; but a number of others also fre- 

 quently occur, as ^, 2t> M' M' ^^- Other 

 varieties met with are \, f, f , f j, ^, -—, &c. ; 

 also |, |, f, |, Yij, |y, &c. ; as also others of a 

 branch with the rarer occurrence. These become more compli- 

 bases of tiie leaves cated as the number of leaves, &c., in the 

 of a kind of Carex, ... -,■,.• .^ ^ 



showing the tristi- spire IS increased ; but m those cases, where 



^?^\ ^^ three- the leaves, &c., are so numerous as to be close 

 ranked arrange- , , ^J • ^i c< • i.i -n- 



meut. to each other, as m the bcrew-pme, the rme- 



Fig. 265. 



Fig. 265. Portion of 



