Page:The Moon (Pickering).djvu/81

 CHAPTER VIII the differences in the atmospheric conditions of the Earth and Moon render it impossible that similar organic forms should exist upon them, the differences, nevertheless, between the two are less marked than those which exist above and below the surface of the ocean—differences which certainly do not serve as a hindrance to a luxuriant organic growth in either region. If the Moon possesses an atmosphere containing watervapottr among its ingredients, no matter how rare it may be, there is no reason in the nature of things why organic growth upon its surface should be impossible, although it seems probable, under these circumstances, that any such growth would be of a low order as compared with that existing under the more favourable conditions upon the surface of the Earth. Moreover, if we find evidence of such growth, this, in its turn, increases the evidence in favour of the existence of water-vapour, and, consequently, in its frozen form, of hoarfrost.

We have already seen that in some localities the Moon's atmosphere may contain as much carbonic acid gas (which is to plants what oxygen is to animals) as our own. How vegetation can exist without water in the liquid state, however, seems at first a more difficult question to answer than how it can exist in a rare atmosphere; but even here we find partial analogies upon the Earth. That certain forms of desert vegetation can go for several years without water is well known, but whether they could continue to grow if the supply in the liquid form were absolutely cut off and only water-vapour furnished them is perhaps doubtful. On the antartic continent a certain kind of lichen is said to exist where the temperature rarely if ever reaches 32°—the melting point of ice. This probably represents pretty closely the condition of affairs upon certain parts of the Moon, and it is possible that water- vapour or hoarfrost deposited upon the vegetation is sufficient to supply all its needs.

Looking at the matter now from another standpoint, we find that the lunar vegetation would have two distinct advantages over our own. In the first place, since the force of gravity is less upon the Moon, the same leaves or fronds or branches would require but one-sixth the effort to lift and support themselves that would be necessary were