Cuar. XV.) ON THE DROSERACEZ. 295 
weight or impact of drops of rain, or by blasts of air. This 
may be accounted for by supposing that these plants and 
their progenitors have grown accustomed to the repeated 
action of rain and wind, so that no molecular change is thus 
induced; whilst they have been rendered more sensitive by 
means of natural selection to the rarer impact or pressure 
of solid bodies. Although the absorption by the glands 
of Drosera of various fluids excites movement, there is a 
great difference in the action of allied fluids; for instance, 
between certain vegetable acids, and between citrate and 
phosphate of ammonia. The specialised nature and_per- 
fection of the sensitiveness in these two plants is all the 
more astonishing as no one supposes that they possess 
nerves; and by testing Drosera with several substances 
which act powerfully on the nervous system of animals, it 
does not appear that they include any diffused matter 
analogous to nerve-tissue. 
Although the cells of Drosera and Dionea are quite as 
sensitive to certain stimulants as are the tissues which 
surround the terminations of the nerves in the higher animals, 
yet these plants are inferior even to animals low down in the 
scale, in not being affected except by stimulants in contact 
with their sensitive parts. They would, however, probably 
be affected by radiant heat; for warm water excites energetic 
movement. When a gland of Drosera, or one of the filaments 
of Dionza, is excited, the motor impulse radiates in all direc- 
tions, and is not, as in the case of animals, directed towards 
special points or organs. This holds good even in the case 
of Drosera when some exciting substance has been placed at 
two points on the disc, and when the tentacles all round are 
inflected with marvellous precision towards the two points. 
The rate at which the motor impulse is transmitted, though 
rapid in Diona, is much slower than in most or all animals. 
This fact, as well as that of the motor impulse not being 
specially directed to certain points, are both no doubt due to 
the absence of nerves. Nevertheless we perhaps see the pre- 
figurement of the formation of nerves in animals in the trans- 
mission of the motor impulse being so much more rapid down 
the confined space within the tentacles of Drosera than else- 
where, and somewhat more rapid in a longitudinal than in a 
transverse direction across the disc. These plants exhibit 
still more plainly their inferiority to animals in the absence 
of any reflex action, except in so far as the glands of Drosera, 
