How Plants Draw in Various Materials 189 



thin, this warping may be rapid enough to be seen by the eye, 

 and forcible enough to exert a considerable pressure; and ad- 

 vantage of these features is taken by plants, to produce, by aid 

 of suitable mechanical arrangements, adaptive movements of 

 various sorts. Of this nature are sundry hygroscopic movements 

 described elsewhere in this book, the self-planting of some 

 seeds; the creeping of some fruits by the twisting movements 

 of hygroscopic awns; the opening and closing, with changes of 

 weather, of most spore-cases and anthers; and the forcible shoot- 

 ing of seeds by hygroscopically-bursting pods. Man has also 

 taken advantage of this principle to construct instruments, 

 called hygroscopes or hygrometers, for showing or measuring 

 the amount of moisture contained in the air. By suitable mechani- 

 cal arrangements the hygroscopically swelling or shrinking sub- 

 stance may be made to twist a pointer over a graduated scale, 

 to cause suitably-clad little persons to make their exits and en- 

 trances to and from tiny houses, or to produce other visible 

 results having appropriate significance. 



So much for the absorption of water; we turn now to absorp- 

 tion of minerals, several kinds of which are needed for the various 

 processes of metabolism inside of the plant. But the subject is 

 comparatively simple. The plant can absorb only those minerals 

 which exist in solution in the water of the soil, dissolved therein 

 from the rocks or from various fertilizers added by man. And. 

 the minerals enter the plant with the water. In Water-plants, 

 and the simpler sorts of the land, they enter mostly by diffusion 

 from the outside supply, traveling everywhere through the water 

 which saturates the plant. But in the higher plants they are 

 swept in with the current through the hairs, cortex and ducts, 

 from which they pass by diffusion to the places of use. It would 

 seem at first sight that their passage through hairs and cortex 

 would be forbidden by the semi-permeable protoplasmic mem- 

 branes. But semi-permeability is wholly relative, and a given 

 membrane which prevents the passage of the relatively large 



