148 The Living Plant 



streams faster, and plants grow better, in warmth than in cold. 

 Light acts analogously on the cell-contents, and one of the results 

 is the brilliant redness of autumn coloration. In some cases the 

 external factors, especially some chemical substances, act repress- 

 ively on the processes, which explains the action of anaesthetics. 

 Third, the factors, when far too weak to exert even an inductive 

 effect can act in a far more remarkable and consequential manner, 

 for they can then serve as guides, or stimuli, in response to which 

 the protoplasm can send its parts into positions found by past 

 experience to be best for the performance of its functions or avoid- 

 ance of dangers. Thus, light far too weak to be directly useful 

 or injurious to the plant yet serves as a guide whereby stems can 

 grow towards it, leaves across it, and roots away from it, those 

 positions being the most advantageous for the performance of 

 their particular functions. And innumerable other cases of this 

 kind are known, of such interest and importance, however, that 

 they must receive a chapter all to themselves under their proper 

 physiological name of Irritability. It is enough for our purpose 

 at present to make clear the existence of the three-kind relation 

 between protoplasmic activity and the external world. 



One does not go far with his studies upon protoplasm before 

 he begins to take thought of its origin. In one way the prob- 

 lem is simple enough, for all of the protoplasm famihar to us 

 originates obviously in only one way, — by growth and division 

 from other protoplasm through reproduction. It is not so long 

 since even scientific men held the contrary belief, still widely 

 persistent among uneducated folk, that low forms of Hfe could 

 originate anew in slime or other fermentable masses; but later 

 experimental studies, chiefly led by the great Frenchman Pasteur, 

 have shown that in all such cases living germs are present, while 

 if precautions are taken to kill all germs by heat or suitable 

 poisons, then no life appears. Every known case of apparent 

 spontaneous generation having thus been investigated and dis- 

 proved, we infer that probably it does not now occur in our 



