CHAPTER V 
PROTECTION AGAINST HEAT, COLD, AND ANIMALS 
LESSON XIII 
Plant hairs, spines, and secretions 
Materials.—Specimens of mullein, geranium, begonia, 
verbena, catchfly (Silene antirrhinum), cactus, thorny lo- 
cust, thistle, etc. 
1. Spines and thorns.—Note and sketch the various 
kinds of spines and thorns. Test their sharpness and 
strength. Are they on all parts of the plant? Have they 
any definite arrangement? Are strongest spines in places 
exposed to greatest dangers? Show in the case of each 
plant just how the spines may be of advantage. Show how 
‘spines may be especially helpful in the regions to which cac- 
tuses are peculiar. Thorn-bushes frequent roadsides, fence- 
rows, etc. Do you think the success of the bushes in these 
places has any relation to the development of thorns? 
2. Hairs.—Note the general appearance of the hair- 
bearing leaves. Peel off and mount some of the epidermis 
from each and study kinds of hairs, their abundance and ar- 
rangement. Sketch the form of each. Select some of the 
pointed ones and mount together with a pin or needle to see 
their relative sharpness. Sketch. Some of the hairs have 
bulbous tips, and these are usually secretory in function. 
Which plants do you think have hairs that may serve 
as protection against injuries from animals, such as insect 
bites, and which ones may also be protected by the hairs 
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