APPENDAGES TO PLANl'S. 



67 



At Fig. 65, the tliorn {a) remains on the stem, while the 

 bark (l) Jias be'en 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 of 

 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 hiot 



The thorns, in some plants, have been known to disappear by cultivation. Lin- 

 nssus imagined, that in such cases the trees were divested of thek 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 between the teeth and divisions of the leaves of 

 many plants. Glands are composed of a 

 single cell dilated at the apex (as Fig. Q>Q^ a) ; 

 of several cells united together, the upper 

 one being the secreting organ (h) of two 

 secreting cells (c), or of more than two (rZ). 

 The &t{7igs of the nettle are glands, formed 

 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, 

 discharging an acrid fluid that enters the 

 puncture in the skin made by the stiff, 

 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. ]S"ettles are 

 similar in their structure and office to the fangs of serpents. 



e. Scales are membraneous expansions found on the root, 



d. Glands— Stingi—f. Scales. 



3* 



