MOTILE MECHANISMS IN HIGHER PLANTS 46*3 



membrane in its different regions ; in some species of mosses 

 they are very complex, the tips of the teeth bending over into 

 the mass of spores and then, as the degree of moisture again 

 varies, shovelling them out and dispersing them in the air. 



The warping and twisting of timber is also due to structural 

 conditions which involve the unequal capacity for swelling in a 

 given direction possessed by the different elements that go to 

 make up the complex tissue we call wood. There is room, 

 however, for much more detailed investigation in this field and 

 the information would be of practical importance in view of the 

 technical utilisation of timber. 



The forces developed in connection with some hygroscopic 

 mechanisms are very great. The woody fruit of Hura crepitans 

 may serve as an example. The fruit of this euphorbiaceous tree 

 is a beautiful object, somewhat resembling a wheel. The seeds 

 are borne in the outer thicker parts of the radially arranged 

 segments. On the complete ripening of the fruit each segment 

 splits during drying into two parts, which curve apart from 

 each other, the whole process taking place with explosive violence. 

 The fruit is thus shattered to pieces and the woody segments, as 

 well as the liberated seeds, are flung to a considerable distance. 

 So far as the present writer is aware, no measurements have been 

 made of the disruptive force, due to the contraction consequent 

 on the loss of imbibed water ; to his knowledge it is sufficient to 

 break a stout iron wire ring bound round the rim of the fruit. 



3. Cohesion of Water. — A few years ago Dixon and Joly 

 and almost simultaneously Askenasy showed that the property of 

 cohesion possessed by water played a very important part in the 

 physics of the ascent of sap and it soon became evident that 

 mechanisms involving this principle were more widely spread in 

 plants than was at first apprehended ; furthermore, that pheno- 

 mena which had been attributed to other agencies really belonged 

 to this class. It is specially to Kammerling and to Steinbrinck 

 that we owe the discovery that a considerable series of move- 

 ments depend on cohesion and their studies have corrected the 

 older views which regarded them as the result of hygroscopic 

 differences in the membranes. In some instances it seems clear 

 that both conditions really obtain but the effects of cohesion are 

 so far more powerful that the hygroscopic qualities are practically 

 negligible though they have a certain theoretical interest in 

 another connection. 



