344 Comparative Physiology. 



is drawn. This may be, in great measure, taken from the deposits 

 laid up in the albumen the preceding year. Mr. Knight, how- 

 ever, was of opinion that there was a formation of sugar in the 

 ascent of the sap, by the action of the air in the air-vessels. The 

 starch taken up by the ascending sap, and the organised sub- 

 stances in a state of imperfect decomposition, must all be de- 

 composed before they can be assimilated. What can become of 

 them, if not decomposed ? they are not fit for assimilation in an 

 organised state, and must be excreted if not appropriated. A 

 great proportion of carbonic acid is probably formed ; indeed it 

 is the opinion of some that all these substances, and water and 

 ammonia, are decomposed into their elements, before forming 

 the proper juices of the plant. So much chemical action joined 

 to that of vitality must be productive of a constant supply of 

 heat to carry on vegetative life. Cold is said to act principally 

 on animals by the condensed state of the air increasing the ab- 

 sorption of oxygen, and thus causing the chemical to prevail 

 over the vital force. It must also act very much by paralysing 

 and stopping vital actions. The absorption of oxygen in plants 

 is not so great as in animals, but cold must act very powerfully 

 by paralysing vital action when it is in activity, the vital parts 

 being so much exposed to its influence. In the dormant state 

 it cannot be productive of much harm, but when the tissue is 

 young and active, and stopped in the performance of its func- 

 tions, it must get into a diseased state, and become less capable of 

 performing vital actions, when heat again supervenes. The ac- 

 cumulation of nutritious matter also, from the inactive power of 

 the organs, must cause fermentation and corruption of the juices 

 themselves ; the chemical power of the oxygen, prevailing over 

 the feeble vital powers of the plant, may induce the formation 

 of an acrid corrosive matter, which injures in place of nourish- 

 ing, and may end in cancer and death. 



Heat, light, and electricity are essential, but that they will 

 not be sufficient without a proper supply of moisture, even in 

 deep-rooted plants, we have evidence sufficient in the year past, 

 of 1842, in which neither fruits nor roots ripened well, but were 

 generally unsound, and the tissues not ripened. 



On Light as a Vital Stimulus, he remarks " that its action as a 

 stimulus has been much overlooked. Its immediate effects upon 

 the animal system are not so manifest as those of heat, but pro- 

 bably not less important. In the vegetable kingdom its mode 

 of operation is less obscure. The operation of light is so closely 

 connected with that of heat, that it is not easy to say what is 

 due to the one and what to the other. There is scarcely a 

 process in the vegetable economy which does not depend on the 

 stimulus of light. The exhalation of vapour from the leaves, 

 and consequent absorption by the roots ; the decomposition of 

 the carbonic acid of the air, and the reception into the system 



