74 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



frost of the surface scarcely yields to the influence of the solar rays at 

 Midsummer. 



102. It is, for the most part, among the Cryptogamic tribes, the 

 Ferns, Mosses, Liverworts, Fungi, and Lichens, that the greatest 

 power of growing under a low temperature exists ; and we accordingly 

 find that the proportion of these to the Phanerogamia, or Flowering 

 Plants, increases as we proceed from the Equator towards the Poles. 

 It has been estimated by Humboldt, that, in Tropical regions, the num- 

 ber of species of Cryptogamia is only about one-tenth that of the Flow- 

 ering Plants ; in the part of the Temperate zone which lies between 

 lat. 45 and 52, the proportion rises to one-half ; and the relative 

 amount gradually increases as we proceed towards the Poles, until, be- 

 tween lat. 67 and 70, the number of species of Cryptogamia equals 

 that of the Phanerogamia. Among the Flowering Plants, moreover, 

 the greatest endurance of cold is to be found in those, which approach 

 most nearly to the Cryptogamia in the low degree of their development; 

 thus the Glumaceous group of Endogens, including the Grasses, Rushes, 

 and Sedges, which forms about one-eleventh of the whole amount of 

 Phanerogamic vegetation in the Tropics, constitutes one-fourth of it in 

 the Temperate regions, and one-third in the Polar ; and the ratio of the 

 Gymnospermic group of Exogens, which chiefly consists of the Pine 

 and Fir tribe, increases in like manner. Still the influence of a high 

 temperature is evident even upon the Cryptogamia and their allies ; for 

 it is only under the influence of the light and warmth of tropical climes, 

 that the Ferns, the highest among the former, can develope a woody 

 stem, and assume the character of trees ; and it is only there that the 

 tall Sugar-Canes, and the gigantic Bamboos, which are but Grasses on 

 a large scale, can flourish. 



103. It appears, then, that to every species of Vegetable there is a 

 temperature which is most congenial, from its producing the most 

 favourable influence on its general vital actions. There is a considera- 

 ble difference between the power of growing and of flourishing, at a 

 given temperature. We may lower the heat of a plant to such a degree, 

 as to allow it to continue to live ; yet its condition will be unhealthy. 

 It absorbs food from the earth and air, but cannot assimilate and con- 

 vert it. Its tissue grows, but becomes distended with water, instead of 

 being rendered firm by solid deposits. The usual secretions are not 

 formed ; flavour, sweetness, and nutritive matter, are each diminished ; 

 and the power of flowering and producing fruit is lost. We see a diffe- 

 rence in the amount of heat required for the vegetating processes, even 

 in the various species indigenous to our own climate ; thus the common 

 Chickweed and Groundsel evidently grow readily at a temperature but 

 little above the freezing point, whilst the Nettles, Mallows, and other 

 weeds around them remain torpid. But the difference is much more 

 strongly marked in the vegetation of different climates; showing an 

 evident adaptation of the tribes indigenous to each, to that range of 

 temperature which they will there experience. Instead of being scantily 

 supplied with such of the tropical plants as could support a stunted and 

 precarious life in ungenial climates, the temperate regions are stocked 

 with a multitude of vegetables which appear to be constructed expressly 



