128 



THE AMERICAN MONTHLY 



[July, 



and by exactly the same means 

 throughout the whole kingdom. Nor 

 is the theory weakened because hairs 

 often serve other purposes than strict- 

 ly physiological ones, for nothing is 

 more common than for an organ, or 

 part, to become modified or special- 

 ized for an office for which it was 

 not primarily intended. Thus it is 

 by no means strange if hairs, while 

 essentially organs of the physiological 

 process of metastasis, should be util- 

 ized for purposes of protection against 

 cold or dryness, as has been sug- 

 gested by some of the writers to 

 whom I have alluded. I have no 

 doubt that they are in most in- 

 stances correct in their inference 

 that great hairiness or wooliness may 

 be a protection against sudden 

 changes of temperature, or a check 

 upon too rapid evaporation. Never- 

 theless, I cannot find that unusual 

 pubescence is generally associated 

 with a warm, or a cold, or a dry cli- 

 mate, although this last relation is 

 the most plausible. It seems to be a 

 fact, however, that the hairiness of a 

 plant bears some proportion to the 

 general sterility of the region in 

 which the plant grows. In confirma- 

 tion of this I am informed that, in the 

 sterile regions of our western territor- 

 ies, not only do the more hairy spe- 

 cies and genera prevail, but those 

 which are hairy elsewhere are especi- 

 ally so there. 



Mr. Charles Darwin is plainly of 

 the opinion that the saprophytic 

 habit of the so-called insectivorous 

 land plants is the result of a struggle 

 against sterility of soil, but not par- 

 ticularly against dryness ; for while 

 these plants usually inhabit sandy or 

 barren regions, they quite as com- 

 monly grow in very wet places. Such 

 being the case, it seems altogether 

 natural that we should find a general 

 increase of pubescence in the plants 

 of a desert region ; for what is the 

 cause of a most remarkable specializ- 

 ation in the case of the insectivorous 

 plants, may readily be conceived to 

 be the cause of a less remarkable 



modification in the general flora of 

 district. A further fact, strengthei 

 ing this idea, has also been notice^ 

 by Mr. Darwin, in his work on ii 

 sectivorous plants, viz. : that the hig| 

 degree of specialization in the gland^ 

 ular hairs of these plants is associated 

 with smallness of the roots, the latter 

 being in most cases barely of sufficient 

 size to collect water from the soil. In 

 some genera, indeed, as, for instance, 

 in Dionoea, the plant does not seem to 

 depend upon the soil at all, for this 

 plant may be grown epiphytically 

 upon moist cotton or sponge. 



It is well known that roots and foli- 

 age-branches are homologous parts, so 

 that in some plants they may easily 

 be made to exchange places and 

 functions, the branches when planted 

 in the soil running to roots, and the 

 roots when turned upwards developing 

 into foliage. It is therefore not 

 astonishing that, in their natural po- 

 sition the roots and their appen- 

 dages and the leaves and their 

 appendages should maintain a more 

 or less steady equilibrium. When, 

 for any reason, the plant does not 

 find use for the natural root func- 

 tions, we should expect it to transfer 

 them to the leaf if possible ; and 

 where the supply of oxygen and ni- 

 trogen compounds is cut off at the 

 root — as it is in a desert region — it is 

 quite in accordance with analogy that 

 the leaf organs should be unusually 

 developed, for the purpose of in- 

 creasing the supply of oxygen or of 

 nitrogenous compounds through the 

 leaf. This will be particularly the case 

 when the assimilative process is not 

 active, and when the plant must 

 therefore, rely more upon obtaining 

 organic compounds ready-made than 

 upon making them itself. 



Aside from the question of the abun- 

 dance or scarcity of food, the plant's 

 means of appropriating its food-ele- 

 ments must be in proportion to the re- 

 quirements of growth. That is to say, 

 other things being equal, a rapidly 

 growing plant, or part of a plant, is 

 more likely to be hairy than is a slow- 



