152 TRISTICHOUS AND OCTASTICHOUS ARRANGEMENTS, 
A third variety of arrangement in alternate leaves is the 
tristichous or three-ranked ( fig. 291). 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 hike manner, the fifth leaf will be 
over the second, the sixth over the third, and the seventh over 
the fourth and first, thus completing a second cycle ; and so on 
with the succeeding leaves. Here the angular divergence is 4, 
or one turn and three leaves, that is, + of 360°=120°. This 
arrangement is by far the more common one among Monocotyle- 
dons, and may be considered as the most characteristic of that 
class of plants, just as the pentastichous arrangement is of 
Dicotyledons. 
Fia. 292. 
Fie. 293, 
hee 
Fig, 292, Pineapple fruit (Sorosis), surmounted by a crown of empty bracts, 
Fig. 293. Cone or fruit of the Scotch Fir. 
A fourth variety of Phyllotaxis in alternate leaves is the 
octastichous or eight-ranked. Examples 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 2 of the circumference, that is, 3 of 
360° = 135°. 
The above are the more common varieties of Phyllotaxis ; : 
but a number of others also frequently occur, as i sy 33, 21, 
&e. Other yarieties met with are i, 1, 2, <7) sh» ay Ke. ; also 
3, 2, 3, 8, 8, 22, &c.; as also others of a rarer occurrence. 
