] 34 Thomson^s Lectures on Botany. 



circumstances are favourable : hence they are properly called ** the lateral 

 or viviparous progeny of the parent on whose surface they appear." 



The lectures which follow contain interesting descriptions of the sterns, 

 herbaceous roots, hybernaclas, stem bulbs, gems, leaves, caulinar and foliar 

 appendages. The anatomy of leaves is one of the most curious and valu- 

 able portions of the work, and which may be said to be almost all the 

 author's own. The knowledge of the structure of leaves enables us to form 

 a correct idea of the importance of these organs in the economy of plants. 

 We find the vessels which convey the sap from the roots terminating in 

 the leaf, and spreading out their contents through its cells to undergo cer- 

 tain chemical changes, which are essentially influenced by the action of the 

 air and light : we find also a new system of vessels commencing here, which 

 take up again the sap thus converted into proj)er jjiice, and conduct it down- 

 wards, depositing in their course the various secretions formed from it, 

 either in the stem or in the roots, as the nature of the plant requires. 



I cannot do further justice to the lecturer by quotations, fearing I have 

 already engrossed too much of your space ; but I cannot forbeai' remarking 

 on this last, that he is decidedly of opinion that the matured sap descends. 

 That this is agreeable to a very old opinion, and also to a very general belief, 

 is certain ; but I fear that a presumption of the idea has rather retarded, 

 than distinctly advanced, a right knowledge of it. That a retrograde motion 

 has been distinctly observed in the leaf, that there are returning vessels in 

 shoots, and that a French botanist saw the sap descending in the autumn by 

 the vessels of the bark, are all well authenticated facts which it would be 

 petulant to deny ; but that sufficient attention has not been paid to this phe- 

 nomenon, is evident from the different opinions now held concerning it. I 

 can remember when,with very few exceptions, every body believed there was 

 a circulation of the sap (a doctrine now almost exploded), and an annual 

 return of it to the roots ; but now this notion is partly discarded, and is sup- 

 posed to belong to the economy of herbaceous plants only; and, with respect 

 to trees, even the possibility is given up, and instead of the true sap returning 

 to the roots, it is found to be arrested in the stem, and there transmuted 

 into timbei*, or expended in the production of shoots, and leaves, and fruit, 

 in the following year. 



As I do not really understand this part of vegetable economy; nor by 

 what agency a grosser fluid can sink, either, by propulsion or attraction, in 

 such a complex tissue of vessels; nor conceive where the empty cells are, 

 which receive it, either in the trunk or fibrous roots ; I feel bound to con- 

 fess my ignorance, in order that the scientific physiologist should take op- 

 portunity to clear up this (to me) difficult point ; but chiefly, that some of 

 your readers may be induced by experiment and observation to throw some 

 practical light on the subject. 



Should these remarks ever meet the eye of the lecturer, I trust he will 

 excuse the freedom with which I have treated a few of his opinions, 

 and accept my unqualified commendation (such as it is) where so justly 

 doe. My observations are nothing more than a part of what the greater 

 number of your readers know, and their only use is to elicit from them, 

 what but for this call, might lie buried in the unobtrusive modesty, or list- 

 lessness, of the possessors. 



I remain. Sir, yours, &c. 

 ——— Hall, Jan. 19. 1828. Inquisitor. 



