STEMS ee 25 
' The tree therefore spreads. In the Ash, on the other hand, 
the terminal bud does not develop into a flower, but one of 
each pair of axillary buds becomes the flower ; consequently 
continued growth in height is posstbte, and the tree is tapering 
rather than broad. : 
Modifica,  -H¢ branches may be modified into spines, as in 
tions of the Blackthorn ; or into tendrils, as in Virginia — 
Branches. creeper. When examining a plant with spines or 
tendrils, in order to decide whether these structures are 
modified branches or modified leaves, it must be carefully 
observed whether they come off laterally, as leaves do, or in 
the angle which the leaf makes with the stem. In the black- 
thorn, the spines or thorns are all branches; in gorse, some 
are branches, others represent leaves. 
A spine or thorn must not be confused with a prickle. The 
so-called “thorns” of a rose-bush are prickles; they differ 
from thorns in not being developed in any particular order, 
and as far as their origin is concerned they may rather be 
compared with hairs. They are not modifications of branches, 
as the thorn is. 3 
Climbing Lhe Hop and Kidney-beans are plants which 
Stems. climb by means of stems. Darwin has described his 
experiments on the hop. He found that the first two or 
three internodes of a young hop plant were quite straight ; 
the fourth was the first that began to revolve, at first slowly, 
taking twenty-four hours to make one revolution; then 
gradually it went faster, until it accomplished the ninth 
revolution in two and a half hours. He found that in the 
hop three internodes revolved at the same time ; when the 
oldest of these three became too stiff from age to go on 
revolving, the next youngest internode began. In many 
other plants, however, only two internodes revolve together. 
Curiously enough, the revolution is not in the same direction 
in all plants; the growing tip of the stem of hop revolves 
from right to left, like the hands of a watch; so, too, does 
honeysuckle. Kidney-beans and convolyulus revolve in the 
opposite direction. 
