Notes. 
379 
governed by light and gravitation. I noted one especially striking 
series of thorn-branches which were on the lower surface of a hori- 
zontal lateral shoot. The ‘ thorn-internodes * pointed vertically down- 
wards, i. e. at right angles to the shoot; but the remaining internodes 
of these thorn-branches were directed horizontally, that is at right 
angles to the thorn-internode and parallel with the lateral shoot 
and pointing towards its apex. Thus each thorn-branch was 
shaped like an L, doubtless because of the marked positive helio- 
tropism of the later formed internodes. 
Biological Significance of the Thorns. — The thorns are protective, 
and their particular rdle appears to be the defence of the young 
branches which arise in connection with the same leaves as them- 
selves. They accomplish their end by (i) standing out at right 
angles to the stem or having a slightly ascending direction : 
(2) developing in the year of their origin and speedily becoming 
hard and lignified. In accordance with their particular function 
they are absent when there is no axillary branch to protect : for 
example, the bud in the axil of the smaller leaf frequently does 
not develope, and then there is no supra-axillary thorn : neither 
do the buds of the lowest pair of leaves of a shoot grow out, and 
here again thorns are absent. 
The Atrophy of the Leaves. — The unequal size of the two leaves 
at a node is by no means a peculiar phenomenon, but it is interesting 
to find a plant actually in the process of diminishing the size of 
certain of its leaves. In some shoots of Randia dumetorum and 
other species of Randia , no difference exists between the two leaves 
at a node. But on the other hand extremes in the other direction 
exist in which there is marked difference in size and the smaller 
leaf falls off early in life. Connected with the atrophy of the 
leaf is the postponement of the development of the thorn, or its 
total absence, and the frequently permanent dormancy of the axillary 
bud. It is very suggestive that coffee-planters have ascertained by 
experience that the best method of cultivating another Rubiaceous 
plant, the coffee-plant, is to do just what Randia dumetorum seems 
to be aiming at, namely to nip off one leaf from each of the 
successive internodes so as to make the leaf- arrangement alternate. 
The postponement in the development of dwindling organs may 
be also seen in the late appearance of the first pair of leaves on a 
thorn-branch. 
D d 2 
