Notes. 
37 8 
definite period, for this is also the case with the axillary shoots 
which terminate in flowers. But the thorn or thorn-branch has a 
peculiar feature in the pronounced dorsi-ventral nature of its base, 
as exhibited by its hyponasty and epinasty, and by its contour not 
being circular but elliptical with the long axis of the ellipse in an 
antero-posterior plane. 
(iii) The structure of a thorn agrees with that of a shoot. It consists 
of epidermis (or periderm), cortex, vascular cylinder, and pith. But 
there are anatomical differences between the thorn and an ordinary 
shoot. The hairs on a thorn drop off much sooner than they do from 
a shoot, or even from a thorn-branch, and the cuticle thickens more 
rapidly. In the thorn the protoxylem has only a few narrow spiral 
vessels; and the later formed xylem consists of parenchyma and 
radial lines of thick-walled prosenchyma with small pits. In a shoot 
the primary annular and spiral vessels are more numerous and 
slightly larger ; in the later-formed xylem wide vessels are inter- 
spersed amongst the thick-walled prosenchyma-cells which have 
larger pits. The sieve-tubes of the shoot have wider lumina. The 
‘ thorn-internode * stands midway between a shoot and a thorn in struc- 
ture, as there are a few wide vessels amongst the wood-prosenchyma. 
In the young stage there is no apparent difference in the structure 
of a thorn and a thorn-internode already possessing leaves. This 
shows that leaves are not formed on thorns because the vessels 
are wider and more numerous, for these distinctions do not appear 
till after the leaves have been formed. I know no case in which it 
can be so clearly shown that the structure of the wood depends on 
the extent of the assimilating and transpiring surface ; here we can 
compare two identical members formed at the same time and under 
the same external circumstances, for we can examine a thorn 
and a thorn-internode arising at the same node. 
Attention has already been directed to the dorsi-ventral nature of 
the thorns and ‘ thorn-internodes ’ : these members also exhibit re- 
markable peculiarities in connection with the assumption of their 
ultimate position. Under all circumstances the thorn, or the ‘ thorn- 
internode, J assumes a position approximately at right angles to the 
shoot bearing it. Thus these structures assume their position indepen- 
dently of the two great directive influences of light and gravitation; 
and this is the more clearly seen when the thorn becomes a branch, 
in which case the subsequently formed internodes have their direction 
