STIMULATION AND ITS RESULTS 401 
stimuli there is evidence that vegetable protoplasm can 
modify its normal behaviour in other ways when exposed 
to similar influences. This form of 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 certam number of unicellular organisms are strongly 
affected by the presence of free oxygen. The most interest- 
ing case of this sensitivity is that of Bacteriwm termo. 
When several of these plants are placed in a drop of water- 
upona slip of glass, covered, and examined under the micro- 
scope, they are found to collect at the edge of the cover- 
glass. Ifa 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. 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 deter- 
mined that the attractive body is malic acid. When a 
capillary tube containing a weak solution of this substance 
is ingerted into water containing some of the antherozoids, 
26 
