OF VITAL MOTION. 23 



movements of the fugitive hearts of the laticiferous 

 web of vessels. 



These remarks upon the phenomena of cell growth, 

 are intended to serve rather as suggestions than as 

 explanations; but while we would be careful not to 

 assign to them an importance they do not deserve, we 

 must be equally on our guard not to scout the idea 

 that physical agents are concerned in these changes. 

 Heat is undoubtedly concerned, and largely so, and it 

 is to be remembered that the changes in the organic 

 substance accord with the natural law of action of 

 this agent in inorganic substances. 



The phenomena of cell-growth point also to 

 the influence of light as well as heat. This agency is 

 necessary to the free formation of the granules of 

 starch: thus potatoes grown in orchards thickly 

 crowded with trees, are always more watery and less 

 rich in amylaceous matter than those cultivated in the 

 open fields. The growth of the layers which impart 

 hardness to wood requires a free exposure to the in- 

 fluence of light ; and for this reason the timber grown 

 in dense forests is never so firm and hard as that which 

 has been freely exposed to the sun. The granules of 

 the substance allied to starch, which impart colour to 

 plants, depend for their formation upon the same foreign 

 power. The growth of the entire plant is also influ- 

 enced by light, and thus the trunk and its branches 



