INTRODUCTION TO CRTPTOGAMIC BOTANY. 67 



even in the animal kingdom ; nay, they grow occasionally 

 within cavities perfectly cut off, as it should seem, from the out- 

 ward air, or where the structure of the tissues is so close as 

 scarcely to admit of the penetration of any save fluid matter. 

 The species meanwhile have a far greater geographical extent 

 than plants in general ; similar climates constantly afford similar 

 species, though with different species intermixed, of which many 

 are altogether local. In every country they constitute an im- 

 portant element in the number of species ; and in some, as in 

 Sweden, they are four or five times as many in number as the 

 Phsenogams, while in South Shetland, there is but a single 

 Phsenogam, and in Cockbum Island, the nineteen species 

 which form its Flora are all Cryptogams. In the tropics, gen- 

 erally, this proportion decreases, but where there is moisture 

 enough even there the ferns often constitute a strikLug, if not 

 a predominant, portion of the Flora. But not only do Cryp- 

 togams prevail frequently to a very great extent in number 

 of species, but in number of individuals also. The myriads of 

 acres covered with reindeer moss in the North of Europe and 

 Asia, produce a number of individual plants of the same 

 species far exceeding anything that can be compared in that 

 respect amongst Phsnogams. In our own country, Fteris 

 aquilina often excludes almost every other vegetation. 



64. The degree in which some Cryptogams are capable of 

 resisting long-continued drought, is most remarkable, though 

 growing on rocks exposed to a burning sun ; witness the 

 Lichens, whether crustaceous or foliaceous, which revive with 

 the first shower as perfectly as a dormouse waking from 

 its winter's sleep, and the same applies to many mosses. 

 And this, be it observed, is totally different from the case of 

 bulbs, which grow in plains which, during a portion of 

 the year, are arid deserts, and burst forth with the first 

 appearance of the periodic rains ; for, in the one case, the 

 whole plant is exposed, and in the other, a body which 

 performs the functions of a seed, the centre of which is pro- 

 tected by the surrounding envelopes . from the drying action 

 of the air; while many bulbs have the additional protection 

 of a greater or less depth of soil. In these cases, it may be 

 observed that the cells are laden with starch or bassorine; 



