STIMULATION AND ITS EESULTS 393 



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 Myxomycetes. They move slowly 

 towards a watery extract of tan, but retreat from a solution 

 of sugar, glycerine, or certain neutral salts. The zoospores 

 of Saprolegnia 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 hyphae were ramifying, he placed 

 another containing a larger proportion of sugar, and between 

 the two arranged a membrane. The hyphae very soon grew 

 towards the stronger sugar solution, and to reach it pene- 

 trated 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 

 and modify their growth accordingly. Almost all show a 

 peculiar relationship to their substratum, stems growing 

 out from it and roots into it in a direction at right angles 



