PITYLLOTAXIS OR LEAF ARRANGEMENT. 149 
Only one leaf can arise from the same point, but it some- 
times happens that, by the non-development of the internodes 
of an axillary branch, all the leaves of that branch are brought 
close together, in which case they form a tuft or fascicle ( fig. 
288), and the leaves are then said to be tufted or fascicled, Such 
an arrangement is well seen in the Barberry and Larch. That 
fascicled leaves are thus produced is rendered evident by the 
fact that in the young branches of the Larch the internodes be- 
come elongated and the leaves are then separated from each 
other. 
The laws which regulate the arrangement of leaves upon the 
stem have of late years been carefully investigated ; and when 
we consider that all the organs of the plant which succeed the 
leaves are formed on the same plan, and follow similar laws, the 
determination of these laws must be considered to be a matter 
of much importance. It has been supposed by some that the 
Fic. 288. Fic. 289. 
Fig. 288. Fascicled or tufted leaves of the 
Larch. Fig. 289. A portion of a branch 
of the Cherry-tree with six leaves, the 
sixth of which is placed vertically over 
the first. The right-hand figure is the 
same branch magnified, the leaves 
having been removed, and numbers 
placed to indicate the points of their 
insertion, 
arrangement of the leaves varies in the different classes of 
plants : thus, that in Dicotyledons where the cotyledons or first 
leaves which are developed are opposite, the regular arrange- 
ments of the leaves in such plants is to be opposite or whorled 
also; and that when they become alternate, this arises from 
the prolongation or extension of the nodes; while in Mono- 
cotyledons, on the contrary, which have normally but one 
cotyledon, that the regular position of the leaves is alternate, 
and that when they become opposite or whorled, this arises from 
the non-development or shortening of the successive internodes. 
The investigations, however, of Bonnet, nearly a century ago, 
tended to prove that all leaves and their modifications have nor- 
mally a spiral arrangement on the stem ; and he was led to this 
belief by observing that if a line be drawn from the bottom to the 
top of a stem or branch, so as to touch in succession the base of 
the different leaves upon its surface, it would describe a spiral 
