APPENDAGES TO PLANl'S. 
61 
At Fig. 65, tlie tliorn (a) remains on the stem, while the 
bark {b) has been peeled on. In the prickle (c) the whole ap- 
pears separated from the plant. The thorn appears to be an 
imperfectly developed bud, which has become indurated. 
Leaves themselves often become spines by the hardening ol 
their midrib or primary veins, and the diminution or absence 
of parenchyma. Branches are sometimes arrested in their 
progress at an early stage of their development, and do not 
appear beyond the surface of the stem ; they form a hnot 
The thorns, in some plants, have been known to disappear by cultivation. Lia- 
nseus imagined, that in such cases the trees were divested of their natural ferocity, 
and became tame. 
Thorns are now considered as buds, which a more favorable 
situation converts into luxuriant branches ; they have been 
compared to the horns of animals. 
d. Glands are collections of cells forming secretions. In 
many plants the secretions from glands give their peculiar fra- 
grance. Glands are sometimes attached to the base of the 
leaf, and sometimes occur in the substance of leaves, as in the 
lemon and myrtle, causing them to appear dotted when held 
to the light. They are found on the petioles of the passion- 
flower, and betv/een the teeth and divisions of the leaves of 
many j)lants. Glands are composed of a 
single cell dilated at the apex (as Fig. 66, a) ; 
of several cells united together, the upper 
one being the secreting organ ip) of two 
secreting cells (c), or of more than two {d). 
The stings of the nettle are glands, formed A 
of a single conical cell dilated at its base, 
and closed at the apex by a small globular 
button which breaks at the slightest touch, 
discliarging an acrid fluid that enters the 
puncture in the skin made by the stiif, 
hair-like process of the gland. When a 
nettle ^ is grasped with violence the sting is crushed, and no 
pain is caused by the discharge of its secretion. ]N"ettles are 
similar in their structure and office to the fangs of serpents. 
e. Scales are membraneous expansions found on the root. 
d. Glands— Stings- e. Sculea. 
3* 
