THE DEFENCES OF COCK-SPUR THORN. 
243 
light; that is to say, whether the thorns are, in the phrases of 
the physiologist, positively geotropic or negatively heliotropic. 
Only by an exact experimental method would it be possible to 
determine the nature of the influence. I think, however, that 
light is the chief, if not the sole, agent. I have been able to ob¬ 
serve the results of several experiments arranged, as it were, 
by Nature. I note that the thorns on the under side of the 
limbs are sometimes turned away from the vertical. When this 
occurs it is often apparently explained by the light relation. And 
in a few cases I found that thorns which had been densely shaded 
under very near foliage of a neighboring tree had actually turned 
strongly toward the shade, when gravitative influence would have 
produced an opposite effect. The thorns therefore seem to turn 
away from light; that is, to be negatively heliotropic. 
A point in the evolution, or supposed evolution, of the thorns 
has interested me, and I venture to introduce it here. Notwith¬ 
standing some recent advances of moment the modes of evolu¬ 
tion are still very obscure. In spite of uncertainties, however, 
there is a mental satisfaction in finding any signs of the course 
which a particular evolutionary development has followed. Ac¬ 
cordingly I examined, for suggestions, the numerous species of 
Crataegus and the near relatives of Crataegus in the great liv¬ 
ing collections at the Arnold Arboretum, especially the pears 
(Pyrus ), apple ( Malus ), and the quince-like Cotoneasters. From 
them it is possible to argue a plausible history for the splendidly 
finished spurs of Cartaegus Crus-galli and other species of the 
genus. It is to be observed that in this group a differentiation of 
two kinds of stems is common. There are the ordinary running 
stems of indefinite growth, and upon them short side stems. The 
latter together bear most of the foliage; the internodes are very 
short and compressed, and the elongation is strictly limited. 
They are hard and rigid, and late in the season—at least in some 
species—are capped by small hard pointed buds, so as to be de¬ 
cidedly sharp in the hand-grasp. They are common in Pyrus and 
Malus. Then in some species there are two kinds of limited 
lateral stems, one like those just described, ending each year of 
active growth in a leaf-bud, another of one or two year’s growth 
