Cs 434 eo a tructural and Ph ysiological Botany. 
‘tation. Each of these floras is again subdivided into zones, 
na the Agate of Magellan, and by the heaths of the Cape of Good a ’ 
‘Hope and of western Europe. . Bae ta 
Since one of the main problems of phyto- geography j is Bs inves- ah 
tigation of the means by which a plant ensures its existence, a physio- ne 
logical system may be contrived, according to the plan prosetem ‘byke 
Humboldt, in which those organs of a plant which serve for the main- ~ 
tenance ofits life—in other words its habit—are first taken intd account ; ~ of 
these characters determining to a great extent the natural features Of. a 
the vegetation. A certain connection can indeed usually be recognised vie 
between the external form of the vegetation and the climatal conditions. o 
which determine its geographical distribution. _On this plan the vege- 
table kingdom may be divided into the seven following classes :— 
1. Woody plants ; 2. Succulent plants, such as the cacti ; 3. Climbing _ : 
plants. (lianes) ; 4. Parasites or epiphytes, like the mistletoe ; 5. Her- 
baceous plants; 6. Grasses; 7. Cellular plants ; and each of these- | Se 
groups may be divided into a greater or smaller number of sections. | 
In order to obtain an idea of the entire vegetation of 
the globe, the surface of the earth has been divided by — 
Grisebach into twenty-four natural floras, or vegzons of vege- 
the phyto-geographical character of which is determined by 
its elevation above the sea-level ; a succession of zones being | 
thus obtained, until at length the line of perpetual snow 
sets a limit to almost all vegetable life. See 
The determining conditions and the actual bourne | 
of these floras are, however, at present but imperfectly 
known, [and, indeed, any such division must be considered — 
as very inadequately representing the endless variety and 
complexity of nature. In all such classifications thereisa 
definiteness and sharpness of outline, which is really an 4 
inherent logical defect of all classifications of natural - 
phenomena, rather than a representation of anything that 
has an exact counterpart in nature]. The districts charac- — 
terised by the occurrence of similar forms and often identical 
species, occupy as a rule lower zones of elevation sis: 7 
ed 
a 
