402 VEGETABLE PHYSIOLOGY 
they make their way very quickly to the orifice of the tube. 
They are very sensitive to the presence of the acid, being 
guided apparently in their movements by very slight 
differences of concentration. When the acid exceeds a 
certain strength they avoid it as earnestly as they seek it 
when it is in greater dilution. 
In the case of the Mosses the attractive substance is 
cane-sugar. Alkalies in any degree of concentration repel 
the antherozoids of both groups. 
A similar sensitiveness to chemical stimulation marks 
the plasmodia of the Myzomycetes. They move slowly 
towards a watery extract of tan, but retreat from a solution 
of sugar, glycerine, or certain neutral salts. The zoospores 
of Saprolegma are attracted by a solution of extract of 
meat, 
The sensitive tentacles of Drosera can respond not only 
to contact, as already described, but also to various sub- 
stances placed upon the leaf. They are easily induced to 
bend by drops of liquid containing protein matter, such 
as solution of albumin, or milk. Certain inorganic salts, 
especially carbonate of ammonia, produce the same effect. 
A curious instance of this kind of irritability has been 
put on record by Miyoshi. He cultivated certain fungi in 
gelatin containing a small proportion of sugar. Under the 
stratum in which the hyphe were ramifying, he placed 
another containing a larger proportion of sugar, and between 
the two arranged a membrane. The hyphe very soon 
grew towards the stronger sugar solution, and, to reach it, 
penetrated the membrane. 
Other instances of similar behaviour might be quoted. 
To this form of sensitiveness the name of chemotaxis has 
been given. 
A few other forms of irritability have been observed in 
various plants. Certain plants growing in currents of water 
take up a definite position with regard to the direction 
of the current, some growing with: it, others against it. 
Certain plants appreciate small differences of temperature 
