CH. V, 5] OSMOTIC PROCESSES 235 



as the concentration of the photosynthetically-formed sugar 

 approaches a quantity which might exert injurious action on 

 the cell, the surplus is converted automatically into starch. 

 The insoluble proteins found abundantly in sieve-tubes have 

 presumably a like explanation, as has the cane sugar found 

 in some leaves intermingled with grape sugar, for cane sugar, 

 weight for weight, exerts only about half the osmotic pressure 

 of grape sugar. In this latter fact, indeed, is probably found 

 the reason why cane sugajr is so much more common a storage 

 form than grape sugar, as Sugar Cane, the Maple tree, and 

 Sugar Beets illustrate. The fact that such changes, easily 

 effected by plants, can produce so great a difference in osmotic 

 properties may help to explain how the water is released 

 from the cortical cells of the roots (page 229) . 



A striking and important feature of osmotic phenomena 

 in plants is this, that the living protoplasm lining the 

 cells can act either as a permeable membrane, permitting both 

 water and dissolved substances to pass, or as a semi-perme- 

 able membrane, permitting only water to pass, or can act 

 at one time as one kind and at another as the other. These 

 various movements, complicated by the nature of the 

 many chemical substances present, and by special phenom- 

 ena of diffusion, solution, imbibition, and like molecular 

 processes, explain, on a purely physical basis, many of the 

 most important phenomena in plant physiology. 



Aside from the living plant, many osmotic phenomena in 

 plant tissues are familiar in our daily experience. When 

 shrunken currants or raisins are immersed in water, es- 

 pecially if heated in cooking, they swell tensely, for there 

 is sugar in their cells. Berries cooked with little sugar swell 

 and burst (though expanding air confined in the tissues also 

 plays a part) ; but cooked with much sugar, as in preserving, 

 they collapse. Dry sugar placed on fresh strawberries soon 

 becomes a sirup, while the berries soften and shrink. The 

 osmotic explanations are all obvious. We place cucumbers 

 and celery in cold water to crisp them, that is to make their 



