140 



OKGANOGRAPHY. 



Fig. 267. 



vertically over the first, and thus 

 completes the cycle, which liere 

 consists of but two leaves; the 

 fourth leaf again is over the se- 

 cond, and the fifth over the third 

 and first, thus completing a se- 

 cond cycle ; and so on with the 

 successive leaves. Here one turn 

 completes tiie spiral, so that the 

 angular divergence or distance 

 between two successive leaves is 

 ^ the circumference of a circle, 

 360°-=-^= 180°. This arrangement 

 is the normal one in all Grasses, 

 and many other Monocotyledonous 

 and the Lime-tree, and 

 other Dicotyledonous Plants, exhi- 

 bit a similar arrangement of their 

 leaA^es. 

 A third variety of arrangement in alternate leaves is the 

 tristichous or three-ranked {Jig. 2G8). Thus, if we start -with 



Fig. 267- Portion of a branch of Plants 

 a Lime-tree, with four leaves 

 arranged in a distichous or 

 two-ranked manner. 



Fig. 268. 



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 com- 

 pletes 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 with the 

 succeeding leaves. Here the angular di- 

 vergence is \, or one turn and three leaves, 

 that is 360° -^^=: 120°. This arrangement is 

 by far the more common one among Monoco- 

 tyledonous Plants, and may be considered as 

 the most characteristic of that class of plants, 

 just as the pentastichous arrangement is of 

 Dicotyledons. 



A fourth variety of Phyllotaxis in alternate 

 leaves is the actus liclious or eight-ranked. This 

 Fig.iCA. Portion of occurs in the llollv, the Aconite, &c. In this, 

 L''?'";'M'^'i'' ^''^ the ninth leaf is over the first, the tenth over 



bases of tiic leaves . , i , i \i ^i • i i 



ofakindofCurex. tlic sccoud, the elcvciith ovcr the tlnrd, and 

 diol^'^Ji'^Ke- ^^ °"' '''"'^ taking eight leaves to complete 

 ranked arrange- the cycle ; and, as the s])iral line here makes 

 "•*"'• three turns round the stem, the angular di- 



vergence will be I of the circumference, 360°-Hg= 135°. 



The above are the commoner forms of Phyllotaxy, but a 



