LECTURE XII. 



THE GENERAL EXTERNAL CONDITIONS OF PLANT-LIFE. 



The vital activity of plants is, like that of animals, the result of the co-operation 

 of two factors; the internal structure, or inherited disposition, and the influences 

 operating from without, or stimuli. 



It is of the greatest importance to be perfectly clear on this point ; therefore it 

 may be permitted to make the nature of the two factors mentioned more intelligible, 

 where possible, by the comparison of an organism with a machine. 



The work done by a steam-engine, for example, depends in like manner 

 upon two factors : namely its structure, and the energy which is supplied to it. 

 Upon the form and combination of its individual parts depends what kind of 

 work it can perform — whether, as a locomotive, it will be in a position to draw 

 a train; or, as an engine, to plough an arable field; or, as a spinning machine, 

 to spin yarn out of thread ; or whether it puis a loom in motion, or planes the 

 iron parts for another machine, and so forth. But be the machine constructed 

 as you will, it performs the work intended by the constructor only if it be set in 

 motion; and this, as is well known, is accomplished by the tension of the steam 

 which moves the piston to and fro in the cylinder. 



Moreover, it is not enough merely that the steam is evolved in the machine ; 

 since, so long as it possesses too slight a tension, not sufficient to overcome the 

 friction of the parts of the machine, these latter do not move. The performance 

 of work will only be attained if, by the supply of sufficient quantities of heat, 

 the tension of the steam reaches a magnitude which gives to the parts of the machine 

 the quickening energy by means of which it can perform the work in view. 



The case is similar with the work performed by an organised body, particularly 

 of a plant, or part of a plant. The work which it is able to perform depends es- 

 sentially upon its structure — upon the combination and chemical nature of its smallest 

 parts. Whether these parts will be set in motion, however, and whether in ac- 

 cordance with their nature and combination they will actually accomplish that which 

 they are able to perform, depends entirely upon whether they obtain from without 

 the energy in virtue of which they are put into those forms of movement, the 

 co-operation of which we distinguish as the vital activity of the organism. 



After these consideradons," it is clear that the investigation of the phenomena 

 of life — i. e. Physiology— has to do with two problems : on the one hand, with 



