98 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



and occur whereever water trickles down constantly, or where 

 the soil or rocks are moist, for where there is sufficient 

 moisture, full exposure is by no means detrimental to their 

 growth. If the doubtful genus Lepto7nitus, and its allies 

 which grow on putrid or unhealthy animals, and the so-called 

 Algse, developed in the intestines of Julus, be eliminated, 

 there is scarcely one that can be considered truly parasitical ; 

 or, with the exceptions before indicated, deriving any nutriment 

 from the soil on which it is fixed. 



79. As regards their distribution over the globe, like other 

 Cryptogamic plants, their limits are extensive; nevertheless 

 there are more marked features than might at first be supposed, 

 from the constant motion and the continuity of the medium 

 in which so great a portion of them grow. Many, however, 

 flourish at depths where the warmth of tropical suns, apart 

 from currents, is felt, and in similar temperatures of the sea, 

 there is certainly a greater difference of species than in Fungi 

 or Lichens. Under given atmospheric conditions, neither 

 genera nor species of non-marine Algae vary much ; at any 

 rate, there is a very close resemblance between those on the 

 Indian mountains, and under analogous conditions of climate, 

 though with the intervention of thousands of iniles. Amongst 

 many of the lower Algse, such as Oscillatoria, Calothrix, 

 &c., it is very difficult to speak of species, because the 

 species themselves are often very loosely defined, and it is 

 almost impossible to judge of such productions merely from 

 dried specimens. It is a remarkable fact, however, that, abun- 

 dant as the red-snow is in the European regions, it never once 

 occurred in the whole of Dr. Hooker's extensive journeys 

 amongst the loftiest mountains in the world, though there 

 were Algse, Lichens, and Fungi enough, which could be 

 identified with European specimens. The Desmidiacew of other 

 countries than Europe and North America have been, at 

 present, bvit little studied, nor does it appear that they are, in 

 reality, numerous. Dr. Hooker's Indian collections show but 

 a trace of them. The Diato7)iacece, on the contrary, occur 



at a distance of oue foot, whereas Lepidium sativum is scarcely greened 

 hy the light of two Argand lamps. — Perx. Narr., vol. 1, p. 88. 



