CHAP. II. IRREGULAR METAMORPHOSIS. 
543 
ignorant of the specific causes by which metamorphoses are 
effected. We know that the cellular tissue, and the secreted 
matter or proper juices, are what chiefly manifest their sensi- 
bility of change ; but beyond this we know absolutely nothing 
whatsoever. In this want of information the simplest manner 
of treating this subject is to take the parts of vegetation in 
succession, and to state what is known of the irregular meta- 
morphosis of each. 
The roots and tubers undergo a vast variety of changes: 
some of which are the effects of domestication, and others 
produced in wild individuals. Some grasses, when growing 
in situations more dry than those to which they have been 
accustomed, acquire bulbs; as if laying by reservoirs of 
nourishment to meet the casual want of a sufficient supply 
of food. Other roots sport, when domesticated, into various 
forms and colours ; as is familiarly exemplified in all those 
which supply our tables. In the turnip the form varies from 
spherical to depressed, oblong, and fusiform ; the epidermis 
from white to yellow, purple and green : the same may be 
said of the radish. The celery, the root of which is fibrous 
when wild, produces under domestication a fleshy round root 
like that of a turnip, known in gardens by the name of 
celeriac. The common potato, the colour of which is usually 
yellow, produces a variety deeply stained, not on the epidermis 
only, but through its whole substance, with purple. The 
parsnip varies from fusiform to spherical ; and there are hun- 
dreds of similar cases of which every body must be aware. 
Metamorphosis of the stem is much less frequent than 
those of the root. The stems of the common cabbage are 
naturally hard and stringy ; but in a variety, called by the 
French Chou moelHer, the stem is succulent and fusiform ; and 
in the Kohl Rabi it forms a succulent tumour above the 
ground, in form and size resembling a turnip. In alpine 
situations the stem becomes shortened in proportion to the 
elevation at which it is produced, but it lengthens in low humid 
situations. Domestication has also rendered tall stems mere 
dwarf, and dwarf stems taller ; the common Dahlia, the mean 
height of which may be estimated at six feet, has been reduced 
by cultivation to a stature not exceeding three. Cabbages 
