vii.] ON THE PHYSICAL BASIS OF LIFE. 107 



muscle. But the scheme which is large enough to 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 

 contractility ; and, it is more than probable, that when the 

 vegetable 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 phenomena, at once rare and 

 conspicuous, as those exhibited by the leaflets of the sensitive 

 plant, or the stamens of the barberry, but to much more widely- 

 spread, and, at the same time, more subtle and hidden, manifes 

 tations of vegetable contractility. You are doubtless aware that 

 the common 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 fineness that it readily penetrates, and 

 breaks off in, the skin. The whole hair consists of a very 

 delicate outer case of wood, closely 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 pro 

 toplasm, 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 contrac 

 tions of the whole thickness of its substance pass slowly and 

 gradually from point to point, and give rise to the appearance of 

 progressive waves, just as the bending of successive stalks of 

 corn by a breeze produces the apparent billows of a corn-field. 



But, in addition to these movements, and independently of 

 them, the granules are driven, in relatively rapid streams, through 

 channels in the protoplasm which seem to have a considerable 

 amount of persistence. Most commonly, the currents in adjacent 



