188 
and intensity. As the answers to these different questions neces- 
sarily vary, according to the plant examined, the results obtained 
in each case are given separately. 
For a description of the nature and morphology of the hooks 
see Trevs’s monograph (l.c.). The modes of experimentation 
adopted, were, for the most part, extremely simple. The ex- 
periments were, as a general rule, performed on at least 5 or6 
pairs of hooks; of the individuals of each pair, one being used ~ 
for experimentation and the other for comparison. At regular 
intervals of time, the growth in thickness of the hooks was 
measured by a Zeiss micrometer, the measurements being given 
in hundredths of a millimetre. 
In Uncaria. Olax. Ancistrocladus. Artabotrys. Luvunga. Strychnos. 
the marked primary curvature of the hook is produced by 
normal growth without contact. The hooks curve backwards, 
the ventral surface being concave. When in contact, the primary 
curvature may be more or less increased, owing to the greater 
growth in length of the convex side; or, no change beyond the ~ 
thickening may take place. The latter is sufficient, by diminishing 
the space enclosed by the hook, to enable it to grasp tightly 
and firmly the support it has caught. In all cases, in the quite 
young hooks, no perceptible contact irritability is shewn, until 
the hook is about 1 week old. The hooks of Ancistrocladus. Olat. 
Uncaria and Artabotrys. under normal conditions attain their 
maximum irritability when from 2 to 4 weeks old, after or 
during the 4th week, the irritability commences to decline , 
whilst after the 6% week, no perceptible, or only a very slight 
irritability to contact can be detected. 
The hooks are most irritable shortly after sclerotization has 
commenced and before it has become marked. The development 
of the sclerenchyma commences first on the concave surface; 
the tissue here being somewhat smaller celled and more compact. 
Soon however, the sclerenchyma commences to develop simul- 
taneously on the sides and back of the hook. The wood cylin- 
der may be at first somewhat thicker on the concave side, than 
on the sides or back. In an attached hook, the subsequent growth 
