Notes. 377 
presents the two thorn-branches, above the large leaf, and /. hr . 
the arrested thorn-branch above the smaller leaf. 
(ii) The thorn arises and grows like a shoot. It appears as a 
multicellular outgrowth in the axil of a leaf, and the deeper layers 
of cells take part in its formation. It is only by the subsequent 
intercalary growth of the shoot bearing the thorn that the latter 
assumes its supra-axillary position. The thorn in connection with 
the smaller leaf arises later than that above the larger leaf at a 
node, hence it is not carried so far up the stem by the intercalary 
growth of the stem. The thorn differs from an ordinary branch 
in that it developes at once and does not wait till next season as 
an axillary bud does. It grows out in the form of a long slender 
structure which tapers to a fine growing-point. At first it is strongly 
hyponastic and is directed upwards parallel to the shoot which bears 
it : by subsequent epinastic growth it comes to stand more or less at 
right angles to the axis. A thorn-branch developes in precisely the 
same manner ; but it at length bears a pair of leaves, placed in the 
transverse plane as on ordinary branches, which appear relatively 
late. As is seen in Fig. 4, the first pair of leaves are inserted at 
the top of a long internode, which for brevity I will term the < thorn- 
internode/ whereas an ordinary branch arises as a short broad 
outgrowth on which two leaves appear at once. Hence even old 
thorn-branches can be recognised as such by the long basal inter- 
node ( c thorn-internode ’) with which they commence ; this is clearly 
seen in the second figure. The typical leafless thorn does not 
differ from an ordinary shoot in that it only grows for a brief and 
D d 
