PHYSIOLOGY: 271 
stem of a tree or shrub, formed of the air and ad- 
ducent vessels, which have grown completely hard. 
-It therefore remains fixed, though the bark be taken 
off. That it arises from a want of food is easily 
proved by the cultivation of thorny plants. Most 
species of our fruit trees have thorns, but having 
been supplied in our gardens with extra food, they 
become boughs, and at last disappear entirely. Only 
such plants as the black thorn, which are almost 
covered with thorns, don’t lose them entirely by 
that treatment, though their number is always di- 
minished. 
Nearly the same thing takes place in thorns, which 
are not formed from imperféctly evolved buds, but 
are other parts of plants, changed in their appear: 
ance. Sometimes the petioli of pinnate leaves, when 
they remain after the leaves have dropped off, be- 
come thorns, as in Astragalus tragacantha, and other 
species of that genus. On the peduncles they grow 
larger, sharper, and assume, after the flower and 
fruit have fallen off, the shape of thorns; for in- 
stance, Hedysarum cornutum: or lastly, the stipulae 
become sharp, ligneous, they remain and change into 
thorns, for instance in the Mimosa. Such changes, 
which frequently occur, especially in oriental plants, 
are generally very regular in their recurrence. _ 
§ 266. 
The prickle, (§ 48), is a prolongation of the 
cutis, and can therefore be taken off along with it. 
This consists of reticular, more or less expanded, 
adducent vessels, anda few air vessels, and is covered 
with 
