1881.J 



MICROSCOPICAL JOURNAL. 



129 



ly growing plant or part. On this 

 principle, as I have already men- 

 ticmed, the newest part of a plant, 

 whether leaf-bud or flower-bud, is 

 usually the most pubescent part. 



It is not easy to account for all the 

 differences among genera, as to hairi- 

 ness or smoothness, but I have no 

 doubt that ultimately it will be ascer- 

 tained pretty nearly why each differ- 

 ent genus is or is not hairy ; for I do 

 not think there can be a doubt that 

 every such difference has a reason, 

 founded in general laws, in the main 

 physiological. 



In the insectivorous plants some of 

 the hairs are adapted to a use not 

 strictly physiological, namely, the 

 alluring and capturing of prey. For 

 this reason they have become most 

 developed and modified on the upper 

 instead of the lower side of the leaf. 

 It is not unusual, however, to find 

 organs of every kind which have be- 

 come specialized for peculiar pur- 

 poses ; and an instance, somewhat in 

 the line of the alluring bait of some 

 of the insectivorous plants, is the 

 change which a simple gland, such as 

 we find on many leaves and petals, is 

 supposed to have undergone, in order 

 to produce the nectary of the flower. 



Where hairs have not been modi- 

 fied for unusual purposes, we can 

 see reasons why they should grow 

 most abundantly and luxuriantly 

 on the under side of the leaf. In 

 the first place, the hairs, being organs 

 of the metastatic process, naturally 

 grow, under ordinary circumstances, 

 where that process is best promoted, 

 namely, in the shade, Avhere also, for 

 the same reason, are the veins with 

 which the hairs are connected. But 

 aside from this, it seems to me not 

 merely a fancy that, as the upper 

 side of the leaf is the one where 

 the chlorophyll-cells most naturally 

 abound, it should be free from any or- 

 gans which Avould cut off the full sup- 

 ply of light and heat which is necessary 

 to the operation of their functions. 

 Moreover, since the stomata open in 

 the light, and evaporation is has- 



tened by heat, the under side of the 

 leaf seems to be the better place for 

 them, for there they are less liable to 

 unrestrained action which would be 

 disturbing and dangerous to the plant. 

 Besides this, we can see that it is an 

 economical arrangement that hairs 

 and stomata should exist together, 

 chiefly for the reason already referred 

 to, that the hairs probably help to 

 retain moisture and check too rapid 

 evaporation. 



After this hasty and necessarily 

 imperfect summary of the more gen- 

 eral facts known as to external epi- 

 dermal organs, I wish now to call 

 attention briefly to some facts con- 

 cerning analogous internal organs. 

 These are particularly the internal 

 hairs of some water plants, of the 

 genera Nymphcea and Niiphar. 



If we examine with the microscope 

 a section of the leaf, or the petiole, of 

 a plant from either of these genera, 

 we are at once struck by certain 

 abundant, thickened, branching, uni- 

 cellular structures scattered through 

 the parenchyma and projecting into 

 the intercellular spaces. These struc- 

 tures have long attracted the attention 

 of microscopists, but not until recent- 

 ly have they been distinctly recog- 

 nized as internal hairs. The stan- 

 dard text-books of the microscope, 

 such as Carpenter's and the Micro- 

 graphic Dictionary, refer rather 

 doubtfully to the resemblance be- 

 tween these bodies and the stellate 

 external hairs of Deutzia and Aly- 

 SSU9H, but they generally shyly avoid 

 calling them plainly internal hairs. 

 Some writers speak of them as " stel- 

 late parenchymal cells ; " and on a 

 purchased slide which I own, they 

 are described as " stellate raphides." 

 But out of the confusion and uncer- 

 tainty which has prevailed with re- 

 gard to these structures, there has 

 gradually crystallized a clear and 

 definite recognition of the fact that 

 they really are epidermal organs, ex- 

 actly analogous to the external hairs 

 of terrestrial plants, such as we have 

 been considering. 



