392 VEGETABLE PHYSIOLOGY 
sensitiveness is less widely distributed than those which 
we have just discussed, but instances of it are fairly 
abundant, especially among the more lowly forms of 
plants. 
A certain number of unicellular organisms are strongly 
affected by the presence of free oxygen. The most inter- 
esting case of this sensibility is that of Bacterium termo ; 
when a number of these plants are placed in a drop of 
water upon a slip of glass and examined under the micro- 
scope, they are found to collect at-the edge of the cover- 
glass. If a small green Alga is placed in the drop of 
water with them, and the slide exposed to light of sufficient 
intensity to enable the decomposition of carbon dioxide to 
take place, the coincident evolution of oxygen attracts the 
bacteria, which at once swarm round the Alga. So sensi- 
tive are they to this attraction, that if the spectrum of 
sunlight is thrown upon the Alga, the bacteria accumulate 
at those parts which are illuminated by the red and blue 
rays, which we have seen to be capable of effecting the 
exhalation of the oxygen. This response to the attraction 
of oxygen is not confined to these bacteria ; it is exhibited 
by many zoospores and also by the plasmodia of some of 
the Myxomycetes. 
When the necks of the archegonia of the Bryophyta 
and Pteridophyta open with a view to the fertilisation of 
the oospheres which they contain, they discharge a certain 
mucilaginous fluid, which attracts to the organ the free- 
swimming antherozoids. Careful experiments have been 
made in many cases to ascertain what is the nature of the 
attraction, and it has been found that the mucilage contains 
various substances which the antherozoids seek. In the 
cases of the Ferns and some Selaginellas, it has been 
determined that the attractive body is malic acid. When 
a capillary tube containing a weak solution of this substance 
is inserted into water containing some of the antherozoids, 
they make their way very quickly to the orifice of the tube. 
They are very sensitive to the presence of the acid, being 
