1881.] 



MICKOSCOPICAL jouknal. 



107 



sects might be made to disclose the 

 habits of the eaters. 



But, as has already been fore- 

 shadowed, it is not only the mere 

 form or figure of the hairs that is 

 characteristic. Their distribution upon 

 different parts of the plants is also 

 a matter of regular and peculiar 

 habit, as well as their existence or 

 non-existence at different stages of 

 the plant's history. 



In many plants the young leaves 

 or stems are hairy or scaly, while the 

 adult parts are quite smooth. 



We recall the fact that the ramenta 

 of ferns commonly fall off when the 

 frond is fully developed ; and this 

 is also true of the hairs of many 

 phanerogams. But even when the 

 hairs are not shed, the adult leaf will be 

 relatively much less hairy than the 

 young leaf, from the fact that the 

 hairs are developed coincidently with 

 the leaf itself, and that their number 

 does not increase as the leaf increases 

 in size. The same number of hairs 

 is, therefore, spread over a much 

 greater surface in the adult leaf than 

 in the young, growing leaf ; and the 

 young leaf is therefore the more pub- 

 escent, relatively to its surface. This 

 has an important bearing, as will 

 be observed when I come to speak of 

 the physiological side of my subject. 

 It follows from the fact I have just 

 mentioned, that to obtain the most 

 striking specimens of vegetable hairs, 

 the microscopist generally selects the 

 young, terminal or rapidly growing 

 leaves, stems or petioles. 



Another point as to distribution, I 

 have barely referred to already, and 

 that is that even when leaves are en- 

 tirely hairless the calyx maybe hairy. 

 Sometimes this is true of the petals, 

 and often of the filaments. In other 

 words (to generalize these facts), hairi- 

 ness begins at the growing point, and 

 persists there the longest. It may never 

 exist elsewhere on the plant to any 

 considerable extent, but it is likely to 

 exist upon the part last developed, 

 whether leaf or flower. The hairs 

 may drop off from the older parts. 



but they are not often absent from the 

 younger parts. This matter also has 

 an important connection with physi- 

 ological facts, which I shall mention 

 hereafter. 



One of the most obvious facts 

 connected with the distribution of 

 hairs is that they are found more fre- 

 quently and more abundantly upon 

 the under side than upon the upper 

 side of the leaves, the sepals or the 

 petals. This is a very general rule, 

 though there are exceptions to it 

 which seem to indicate that the rela- 

 tive hairiness of the two surfaces of 

 the leaf depends upon the position in 

 which the leaf grows. I am inclined 

 to the opinion that when the leaf 

 naturally assumes such a position 

 that one side receives a much larger 

 amount of light than the other, that 

 side will be the least hairy, if not en- 

 tirely destitute of hairs, while the sha- 

 ded side will be the one most favorable 

 to hairiness. In plants whose leaves 

 grow more or less perpendicularly, 

 the difference between the two sides 

 is apt to be less marked. 



Heliotropism may affect the rela- 

 tive hairiness of the two sides of the 

 leaf, either in causing a greater differ- 

 ence by presenting one side more 

 constantly to to the light, or in caus- 

 ing less difference by presenting both 

 sides equally. It is said that the 

 " compass plant " of our western 

 prairies {Silphium laciniatum), always 

 presents its leaf in a plane with the 

 north and south meridian, and both 

 sides of its leaf are alike ; but, 

 aside from its heliotropism, this may 

 perhaps be due merely to the leaf's 

 tending to a perpendicular position. 



In the Bayberry {Myrica cert/era) 

 we find that the upper side of the 

 leaf is the more hairy, while the 

 glands follow the general rule in being 

 more abundant on the under side. 

 But, notwithstanding these occa- 

 sional exceptions (which will doubt- 

 less be found to be explainable in 

 accordance with the general theory), 

 it is a rule that the under side of the 

 leaf, and that which corresponds to 



