APPENDIX. 



9, in p. 4. — In comparing plants with animals, the leaves can only be compared 

 to lungs ; and, similarly to lungs, it is true, they aerate the sap, and imbibe oxygen, 

 as the lungs do to the blood : but, when we carry the comparison further, we find 

 that not only do the leaves imbibe oxygen, but they also, by imbibing the chemical 

 power of the light, decompose carbonic acid, absorbing the carbon, and setting the 

 oxygen free. This is a power which has never been ascribed to lungs ; and, as the 

 chemical power absorbed probably acts m other ways on the sap presented (see 

 124), though it is difficult to discriminate between organic secretion of particular 

 organs and the chemical power of light, it has been by many eminent physiologists 

 called digestion. Comparative physiology is valuable as assisting us to understand 

 more readily what we are ignorant of, by comparing it with what we are already 

 acquainted with. It is necessary to know the functions which the different organs 

 perform before we can estimate their value, or know the necessity of supplying 

 them with proper food ; and the more we can simplify the subject, by classifying 

 one organ in one organised being with one destined to a similar purpose in another, 

 we the more readily ai'rive at a general knowledge of the whole. There are many 

 difficulties, however, in comparative physiology ; and the proper class of organs to 

 which leaves may belong seems one of the principal stumbling-blocks. 



103, in p. 26. — It may be questioned whether the roots of Rosaceee, &c., abound 

 in adventitious buds. It is more likely these buds are called into existence by an 

 effort of the vitality of the plant. In such as the Rhus, Papaver, &c., which 

 abound in a thick viscid sap, the very smallest pieces, in which it is scarcely possible 

 buds could be formed, ai-e found to produce them, if they have only fibres to collect 

 nourishment. The buds are generally formed at the edges of the cut, where the 

 leaf is extravasated, showing they are formed from the extra vasated sap, and did not 

 previously exist in the state of buds. The edge of the cut is sometimes so crowded 

 with buds, that they cannot be supposed to have had pre-existence in such large 

 quantities. The buds noticed at 121 maybe more properly called axillary than 

 adventitious. 



128, in p. 34. — It has been customary to call the cause of fruiting an accumula- 

 tion of nutritive matter. Were this the case, we would add to the fruitfulness of a 

 tree by augmenting the quantity of its food or nutritive matter. The reverse of 

 this, however, more often takes place, as in ringing and taking away roots, impo- 

 verishing the soil, &c., all which diminish the quantity of nutritive matter, and yet 

 generally add to fruitfulness. It is not that impoverishing is itself the cause : were 

 we able to increase the light and heat as we can increase food, there would be less 

 cause for impoverishing. The supply of food, however, is most at our command; 

 the others, especially the light (the most needful), we have but little power over, 

 and must, therefore, curtail the food to suit ( ur limited means. A certain highly 



