68 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



influenced in this manner, appears from the fact, that, by reflecting 

 light upon germinating seeds, in such a manner as that it shall only 

 fall upon them from below, the stems are caused to direct themselves 

 downwards, whilst the roots grow upwards. There can be no doubt, 

 however, that Light has also a more direct influence on the develop- 

 ment of particular organs in certain Vegetables. Thus when the gem- 

 mules* of the Marchantia polymorpha (one of the ffepaticce or Liver- 

 worts), are in process of development, it has been shown by repeated 

 experiments, that stomata are formed on the side exposed to the light, 

 and that roots grow from the lower surface ; and that it is a matter of 

 indifference which side of the little disk is at first turned upwards, 

 since each has the power of developing stomata, or roots, according to 

 the influence it receives. After the tendency to the formation of these 

 organs has once been given, however, by the sufficiently prolonged in- 

 fluence of light upon one side, and of darkness and moisture upon the 

 other, any attempt to alter it is found to be vain ; for if the surfaces 

 be then inverted, they are soon restored to their original aspect by the 

 twisting growth of the plant. 



91. The same amount of this agent is not requisite or desirable for 

 all Plants ; and we find in the different habitats which are characteristic 

 of different species, even amongst our native plants, that the amount 

 congenial to each varies considerably. Generally speaking, the succu- 

 lent thick-leaved Plants require the largest amount ; their stomata are 

 few in number ; and the full influence of light is requisite to induce 

 sufficient activity in the exhaling process ; accordingly we find them 

 growing, for the most part, in exposed situations, where there is nothing 

 to interfere with the full influence of the solar rays. On the other 

 hand, plants with thinner and more delicate leaves, in which the ex- 

 haling process is easily excited to an excessive amount, evidently find 

 a congenial home in more sheltered situations ; and there are some 

 which can only develope themselves in full luxuriance in the deep shades 

 of a plantation or a forest. By a further adaptation of the same kind, 

 some species of Plants are enabled to live and acquire their green colour 

 under an amount of deprivation which would be fatal to most others ; 

 thus in the mines of Freyburg, in which the quantity of light admitted 

 must be almost infinitesimally small, Humboldt met with Flowering 

 Plants of various species ; and Mustard and Cress have been raised in 

 the dark abysses of the collieries of this country. 



92. Generally speaking, however, the Cryptogamia would seem to 

 be better adapted than Flowering Plants to carry on their vegetating 

 processes under a low or very moderate amount of this agency. Thus 

 Humboldt found a species of sea-weed near the Canaries, which pos- 

 sessed a bright grass-green hue, although it had grown at a depth of 

 190 feet in the sea, where, according to computation, it could have 

 received only 1-1 500th part of the solar rays that would have fallen 

 upon it at the surface of the ocean. Many Ferns, Mosses, and Lichens 



* These gemmules are analogous to the buds of higher plants ; and they consist of 

 little collections of cells, arranged in the form of flat disks ; which are at first attached 

 by footstalks to the parent plant, but afterwards fall off, and are developed into new 

 individuals. 



