394 VEGETABLE PHYSIOLOGY 



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 proteid 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 hyphse 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 cliemotaxis 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 

 to the surface. This can be seen by cultivating them so 

 that they do not emerge in the normal direction but from 

 the side of a cube of earth. They do not long maintain 

 this direction, as they speedily feel the influence of light 

 and gravity. If however appropriate means are adopted to 

 eliminate these, the growth is always at right angles to the 

 surface of the soil in which they live. 



