202 BOTANY. [CHAP. vii. 



of Algas, and in the sea there are fairly well defined 

 zones of vegetation. As a rule the green and olive- 

 brown groups of seaweeds are most numerous in cold 

 regions, whereas red seaweeds become more abundant 

 as the tropics are approached. The larger highly- 

 organized seaweeds do not grow at a very great depth, 

 but form a fringe round the shores. Amongst Alg89 we 

 meet with considerable variety as regards adaptation to 

 circumstances ; many species are true parasites, cases of 

 mutualism or commensalism between Algae and some of 

 the lower forms of animal life are on record. The 

 dimensions of Alg89 are very various, the diatoms are 

 unicellular and microscopic, whereas Macrocystis pyrifera 

 is frequently five hundred feet in length. The species 

 of Lessonia grow erect with the habit of forest trees, 

 forming in fact submarine forests. 



Ferns occur in all climates between the polar regions, 

 where, however, few species occur, to the tropics, where 

 they are by far most abundant and attain their maximum 

 of development. Many genera are limited to equatorial 

 regions, or extend very little beyond ; on the other 

 hand, few genera are confined to a single continent. 

 The tree- ferns are tropical, although some species extend 

 to Tasmania and New Zealand. On the Andes they 

 grow along with the cinchona trees at an elevation of 

 four to eight thousand feet. The majority of ferns 

 thrive only under a peculiar combination of climatic con- 

 ditions, dry regions producing very few species ; damp, 

 shady places, with a comparatively equable temperature, 

 suit them best, consequently we meet with the greatest 

 percentage compared with the entire flora in insular 

 climates ; in fact, the smaller and more distant from 



