98 ON THE PHYSICAL BASIS OF LIFE 



embrace the activities of the highest form of life, covers 

 all those of the lower creatures. The lowest plant, or 

 animalcule, feeds, grows, and reproduces its kind. In 

 addition, all animals manifest those transitory changes 

 of form which we class under irritability and contractil- 

 ity; and, it is more than probable, that when the vege- 

 table world is thoroughly explored, we shall find all 

 plants in possession of the same powers, at one time or 

 other of their existence. 



I am not now alluding to such phsenomena, at once 

 rare and conspicuous, as those exhibited by the leaflets 

 of the sensitive plants, or the stamens of the barberry, 

 but to much more widely spread, and at the same time, 

 more subtle and hidden, manifestations of vegetable 

 contractility. You are doubtless aware that the com- 

 mon nettle owes its stinging property to the innumer- 

 able stiff and needle-like, though exquisitely delicate, 

 hairs which cover its surface. Each stinging-needle 

 tapers from a broad base to a slender summit, which, 

 though rounded at the end, is of such microscopic fine- 

 ness that it readily penetrates, and breaks off in, the 

 skin. The whole hair consists of a very delicate outer 

 case of wood, closery applied to the inner surface of 

 which is a layer of semifluid matter, full of innumerable 

 granules of extreme minuteness. This semi-fluid lining 

 is protoplasm, which thus constitutes a kind of bag, 

 full of a limpid liquid, and roughly corresponding in 

 form with the interior of the hair which it fills. When 

 viewed with a sufficiently high magnifying power, the 

 protoplasmic layer of the nettle hair is seen to be in a 

 condition of unceasing activity. Local contractions of 

 the whole thickness of its substance pass slowly and 

 gradually from point to point, and give rise to the ap- 

 pearance of progressive waves, just as the bending of 



