1 62 Barber .—On the Nature and Development of the 
beneath the thorns already described, would need a careful 
study of the plants in their native surroundings. It seems, 
however, probable that, in the majority of cases, the corky 
cushions have as their function the retention of the thorns — at 
any rate in the young plant-after secondary thickening has 
commenced. In such scandent thorny plants as Caesalpinia 
Nuga , where such an increase in thickness is inconsiderable, 
the excrescences, although of remarkable length, increase but 
little in thickness with age (Fig. 13). Their presence, in this 
case, by extending (sometimes threefold) the circle of opera- 
tions of the persistent claws, probably renders the thorns both 
more dangerous as weapons of offence, and more powerful as 
organs of climbing. 
Very different is the explanation offered of the presence of 
a corky layer at the base of the thorns in Rosa. Kauffmann, 
to whom belongs the credit of proving that the periblem takes 
part in the formation of Rose-prickles, writes concerning these 
organs : c The cork formed beneath the prickles enables the 
latter to be easily pulled off, or even to fall off of their own 
accord V An advantage to the plant, in this case, would be 
the prevention of a rupture of tissues on the forcible separation 
of the thorns. It is also conceivable that thorns, thus readily 
separable, may penetrate climbing animals, and be borne away 
by them ; and in that case the contrivance may be analagous 
to the extreme brittleness of the spines of Opimtia. 
Of a somewhat similar nature to that in Rosa would appear 
to be the cork formation in Robinia Pseudacacia. Mittmann 
describes it thus : ‘ The thorns become dried up at the end of 
their first period of vegetation, and become separated by a 
corky layer from the underlying tissues ; they remain, how- 
ever, attached to the tree for several years 1 2 .’ 
Whatever the function of these corky layers may be, there 
can be no doubt as to the significance of the tough cushions 
at the base of each bunch of Cactus- spines. Delbrouck has 
1 Kauffmann, Ueber die Natur der Stacheln, Bull. Soc. imp. nat. de Moscow, 
1859. 
2 Mittmann, 1. c. 
