249 



influence ; yet we must, at first, keep more to that arrangement, since 

 it is strictly logical, which will remain fixed for incalculably long 

 time, while, of course, the last arrangement only holds good for the 

 existing condition of science. When, namely, we review the various 

 influences upon which the life and healthy vegetation of a plant are, 

 according to our present physiological knowledge, dependant, we 

 quickly find that only a small number of physical forces are as yet 

 detected by us, in their action upon the organism, that on the other 

 hand, a proportionately large number at present altogether baffle our 

 endeavours after a more accurate comprehension of their action, 

 although we may safely assert that the life of the plant is, and must 

 be, as much dependant on them as upon the others. Merely by way 

 of example, I will mention light, electricity, and the pressure of the 

 atmosphere. The two first, as continually in action in every chemical 

 process; the last, of essential importance in all the processes and re- 

 lations between gases and vapours; must likewise powerfully affect 

 the life of the plant, which consists in progressive chemical combina- 

 tions and separations, in continual absorption and excretion of vapours 

 and gases. The how is as yet a complete mystery to us, and many 

 of the at present wholly incomprehensible conditions in extension 

 and distribution, may sooner or later find sufficient explanation in 

 these influences." — p. 242. 



In his remarks upon " the Dependance of the Distribution of Plants 

 on Physical Conditions," from which the above extract is taken, the 

 author gives a graphic sketch of the various appearances successively 

 presented to the eye of the botanical geographer as he scans the ve- 

 getation of the globe, from the " snow-covered ice-plains of the ex- 

 treme North, where the red-snow Alga alone reminds us of the exist- 

 ence of vegetable organization," in a southward direction, down to the 

 garden of Orotava, in Teneriffe, where the gigantic arborescent lily- 

 like Dracaena " recounts to the musing listener the traditions of 

 thousands of years." The six zones of vegetation thus passed 

 through have presented us, in conjunction with the continually in- 

 creasing temperature, a continually differing and ever a more luxuri- 

 ant vegetation. Ascending the Pic of Teyde, and counting by the 

 limits of vegetation, we may then re-survey in a few hours' climb, the 

 wide journey from Spitzbergen to the Canaries, an extent of upwards 

 of fifty degrees of latitude. In our ascent of the Pic we find that 



" Man has taken possession of the soil of the plain at its foot, and 

 dislodged the original vegetation. Through vineyards and maize- 

 fields we ascend, till the shades of the evergTeen bay-laurel surround 



