Thomson^ s Lectures on Botany. 129 



animals, vegetables, and minerals ; the difficulty of ascertaining where the 

 animals end, and the vegetables begin ; and whether the polypi (compound 

 or divisible animals) should rank as animals, or be considered as the highest 

 grade of vegetables ; mentions the various definitions given of vegetables, 

 and offers one which he considers the least objectionable, viz. " A plant is 

 a living organised body, which requires food and air for its support, groivs, 

 propagates its species, and dies ; and differs from animals in being nourished 

 by co7itinued absorption by its external surface.''' This definition is sufficiently 

 comprehensive and just; and had the word "absorption," which here infers 

 an active rather than a passive principle, been changed into one implying 

 that a plant is a recipient only, the definition would, perhaps, have been as 

 correct as it is concise. 



The lecture then proceeds " with a general view of those functions which 

 plants possess in common wdth other organised bodies," i. e. with animals. 

 A plant is composed of solid and fluid parts ; the former comprehending 

 fibres, which are endowed with elasticity and contractibility, and vessels in 

 which fluids move. Those of the same species have always the same parts 

 of structiire, which are evolved in the same manner ; and, except the varia- 

 tions which soil, situation, or seasons produce, their specific characters 

 remain unvariable. Although the vitality of vegetables cannot be defined, 

 yet we know it, when present in any object, by its effects, and it may be said 

 to be " that property of matter connected with organisation, which animals 

 and plants possess in common, of continuing life." Both animals and vege- 

 tables continue it to their offspring : by viviparous animals, before its 

 expulsion from the uterus ; by the oviparous, to the punctum saliens (pro- 

 truding point), which is afterwards the chick, before the egg receives its 

 shell ; and by vegetables, to the embryo contained in the seed, long before 

 it is separated from the plant. Vitality is fugitive when exposed to unfa- 

 vourable concomitants, and wonderfully existent in situations where neither 

 tlevelopement nor decomposition can take place. The particular functions 

 of vegetables which depend on vitality, are the germination of the seeds, 

 the growth of the plant, its absorption, respiration, perspiration, adaptation 

 to climate, and resistance to cold. The progressive growth of a plant de- 

 pends on its vitality ; it selects (receives) its nourishment, digests, assimi- 

 lates, and deposits in its body those secretions which characterise its kind. 

 The simple absorption (reception) of the fluids by the root, and " the circu- 

 lation or rather progressive motion" or ascent of the sap, depend on its vital- 

 ity; the sap continues to rise during the life of the plant, which, if almost 

 dead from want of water, recovers on receiving a supply ; but, if the vitality 

 be extinct, the corpse may imbibe, but cannot be restored again by water. 

 " Did the ascent of the sap depend on any other circumstance than the living 

 action of the vessels, it would only be necessary, in order to restore any 

 decayed plant, to supply it with moisture in a proper temperature." 



All this is very true, and satisfactorily proved ; but we should take along 

 with us, that vitality is only one of the properties of a plant, and that 

 without the stimulus of external influences it remains dormant, though not 

 extinct. We cannot see active vegetable life, without also observing the 

 ascent or motion of the sap ; but we often witness the ascent of fluids, 

 without vitality : for instance, the syphon, and the effect of " capillary attrac- 

 tion," as it is called, by which a cistern or tub of water may be emptied by 

 merely a bit of list. This, however, is not mentioned as a correction of 

 what is advanced by the lecturer, but only, that while the functions of 

 plants are under consideration, we should look to the exciting causes, as 

 well as to the effects thereof, on the vegetable constitution. 



" To the living powers of vegetables we must also revert, to account for 

 the changes of the sap into the solid components and peculiar juices of the 

 plant. No mechanical principles can produce those effects ; they are op- 

 posed to the chemical affinities which exist between the materials com- 



Vol. IV. — No. 14. k 



