336 Comparative Physiology. 



stigma to a branch. The difference however is, perhaps, as 

 much produced from the different quality of the food enabhng 

 a different form to be developed and vice versa, as from a 

 stimulus given by the food. The production, or capability of 

 production, of a new being from the bud seems only a lower 

 grade of the function of reproduction, than the more perfect 

 form of it in producing a seed. The changes produced by the 

 different nature of the food in the lowest grades of plants, pro- 

 ducing sometimes a lichen, sometimes a conferva, from the same 

 germ, according to the absence or presence of water, seem 

 apparently to countenance the idea of there being something 

 equivocal in their developement, perhaps more than in their 

 generation. The changes produced on higher grades of plants, 

 by the different nature of their food, have frequently been 

 found to alter them so mvich as to cause them to be reckoned 

 distinct species ; yet it has been found that the seedlings from 

 these plants resume the ordinary habit of the species, when again 

 under ordinary circumstances. There may be much of this in 

 the changes of plants so nearly resembling each other as confervae 

 and lichens ; and a fixed character in the germ is more indicative 

 of purpose and wisdom, till the equivocal has been more cer- 

 tainly determined. 



On Heat as a Vital Stimulus, he remarks that " all vital 

 action requires a certain amount of caloric for its due per- 

 formance, and can only continue within a certain range of 

 temperature. The greater the amount and variety of vital 

 action, the more immediate is the dependence of the individual 

 on the maintenance of its usual temperature. Plants are almost 

 entirely dependent on the medium they inhabit for the neces- 

 sary supply of caloric ; and their vital actions are so adjusted as 

 to be carried on within very wide extremes of heat and cold. 

 In the Chinese embassy, a species of Marchant^« was found at 

 the Island of Amsterdam, growing in mud hotter than boiling 

 water, at a hot spring ; and the beautiful Protococcus nivalis, or 

 red snow, reddens extensive tracts in the arctic regions, where 

 the perpetual frost of the surface scarcely yields to the in- 

 fluence of the solar rays at midsummer. The stimulating 

 action of heat is very obvious on plants ; it increases evapo- 

 ration by the leaves, and consequently absorption by the roots, 

 supplying the water which prevents its tissue from being dried 

 up, and, by its conversion into vapour, moderating the tem- 

 perature, which would otherwise be excessive. If the supply of 

 water is deficient, the tissues get dense and contracted ; the 

 shrubs in sandy Eastern deserts assuming a stunted and prickly 

 appearance. Cold depresses vital action, and, if very severe, con- 

 geals the juices, and bursts the vessels ; the viscidity of the fluids, 

 and the slow conducting power of the wood, tend to resist this 



