Comparative Physioloyy. 389 



retarding' nutrition we thus increase the tendency to rej^roduc- 

 tion ; and, vice versa, by picking off flower buds, and increasing 

 food, we increase the tendency to nutritive growth. 



Of the manner in which roots, buds, and shoots are produced 

 we know but little in a general way, unless that accumulations 

 of matter, especially vascular, favour their production. It is 

 generally stated by physiologists, that a single cell, in a proper 

 situation and under proper circumstances, is all that is neces- 

 sary for the preparation of a bud, the nucleus of all growth. 

 Some have thought them connected with the medulla : Mr. 

 Knight thought they were from the alburnum. Masses of buds, 

 however, may be seen in many cases generated in enormous 

 quantities, and crowded together without normal ordei", from 

 extravasations of sap, on all parts of a plant. 



Of that mysterious power which guides the development of 

 the plant, evolving the different organs according to the normal 

 manner of the different species, few have attempted to give any 

 definition. As I noticed before, Miiller has likened it to an 

 idea, a picture of the imagination, to which the actions of 

 vitality are constrained to conform, thus causing them to deve- 

 lope after a normal manner, and produce each being after its 

 own kind from the picture. When alterations are made by 

 hybridisation, &c., the picture we can imagine to be conform- 

 ably altered, and we might thus construct a plausible theory ; the 

 great difficulty, however, is to imagine the seat of the sen- 

 sorium where such picture could be formed. Bonnet and other 

 Continental writers have adopted a different opinion, and con- 

 tend that all the parts of a plant are contained in embryo in 

 the original germ ; and that the actions of vitality only cause a 

 developement of previously formed parts, and not a formation 

 of new. Mr. Main, in this country, has been the principal 

 advocate of these opinions. A membrane or indusium, visible or 

 invisible, he says, always surrounds the germ, which contains 

 all the organs of the future plant ; all the parts of the plant are 

 afterwards developed from this indusium, in which they are 

 contained, he says,' in embryo, and developed as the membrane 

 expands ; and it throws off every year a new layer of liber and 

 alburnum, in exogenous plants. As I before noticed, how- 

 ever, if every thing is contained in embryo previously to being 

 produced, it does not account for accidental interference of 

 hybridisation, or adventitious buds ; nor does it allow for leaves 

 and flowers being mutually transformable. It, however, gives 

 a tangible shape to our ideas, by allowing us to conceive of 

 what is invisible, by referring it to something Ave are already 

 acquainted with ; as we conceive of a spirit having bodily 

 organs, though we are onl}^ assisting the imagination to com- 

 prehend, and are entirely ignorant whether we are right or 

 wrong. If such a thing as an indusium, or germinal membrane 



