84 IMBIBITION AND MOLECULAR STRUCTURE 



permanently alter starch-grains, but may also remove from many cell-walls 

 certain constituent substances 1 . Moreover, severe tension may produce 

 mechanical ruptures, as is instanced by the formation of cracks in dried 

 starch-grains, and by the bursting of the outer layers of the cell-wall, when 

 bast fibres swell in dilute sulphuric acid. 



External agencies, the application of which causes no permanent 

 injury, may induce temporary alterations of molecular structure, from 

 which a return to the normal condition is possible. The intensity neces- 

 sary to convert a temporary modification into a permanent change is 

 specifically different for each organism, and for each part of an organism. 

 This is well illustrated by the differences which exist between the resistant 

 powers of different plants to desiccation. Thus certain lichens and mosses 

 and many seeds are able to withstand severe and almost complete desicca- 

 tion, whereas many plants die as soon as the percentage of water present 

 is reduced at all markedly below the normal, and between these two 

 extremes a variety of intermediate stages occur 2 . 



For the maintenance of vital activity, a sufficient supply of water is 

 absolutely essential. The presence of water causes a marked alteration in 

 certain of the properties of bodies which are capable of imbibition and 

 swelling ; thus the cell-wall when saturated with water is soft and pliable, 

 but, like the plasma, becomes hard and brittle when dry. At the 

 same time other physical properties, such as rigidity, elasticity, ductility, 

 and the power of conduction of heat, electricity, &c., undergo a more 

 or less pronounced alteration, which increases progressively with the loss 

 of water. 



The separation of the micellae from each other by the water of imbi- 

 bition gives them greater freedom of movement and more marked powers 

 of adjustment. Thus the cell- wall of Caulerpa, or the thallus of Fucus, 

 which when dry are extremely brittle, may safely be strongly bent when 

 they have absorbed a little water, and when fully saturated may be twisted 

 into a spiral without any rupture taking place. Since such torsion does 

 not cause the exudation of any water, it follows that a redistribution 

 of water must take place in the thallus, a little of the imbibed fluid 

 being transferred from the concave to the convex side. When a dry 

 membrane swells slightly, its power of resistance to stresses and strains 

 increases under normal conditions, but when the swelling is very great, 

 as, for example, when the gelatinous modification takes place, this power, 

 as might be expected, decreases. 



In Cetraria islandica, many algae, &c. The changes observed by Jonas Cohn (Jahrb. f. wiss. 

 Bot, 1893, Bd. xxiv, p. 160) to take place when collenchyma cells are boiled for some time are 

 probably due to a similar action. 



8 Schroder, Bot. Unters. a. Tubingen, Bd. n, Heft i, 1886. Ewart, On the power of withstanding 

 desiccation in plants, Trans. L. Pool. Biol. Soc., vol. xi, 1897, p. 151 ; 1896; and 1894, p. 234. 



