234 A TEXTBOOK OF BOTANY [Cn. V, 5 



plasm until the cell is full-grown. By use of the same power 

 roots force and enlarge for themselves passages through 

 hard soil, even prying aside stones in the process ; and by 

 the same power they disrupt masonry and lift curbstones in 

 streets. So essential is osmotic pressure to growth, and 

 hence so indispensable is adequate water to growing plants, 

 that any marked scarcity of water, or rapid removal thereof 

 from the plant, always checks its growth. This is why the 

 growth rate of a plant always falls, other things being equal, 

 when transpiration becomes active, and vice versa : why 

 plants tend to grow faster at night than in daytime : and 

 why growth usually is checked with the sunrise. 



The question must now occur to the student, whether 

 osmotic pressure can ever become so great as to strain if not 

 burst the plant cells. This does in fact sometimes happen. 

 Thus some fruits, notably Plums, in warm moist weather 

 occasionally burst, from this cause, on the trees. In Tomato 

 plants, watery blisters are sometimes formed osmotically, 

 producing a kind of " physiological disease" called Oedema. 

 Most kinds of pollen (the small yellow grains producing the 

 male cells in flowers), when placed in water, swell and burst, 

 of course to their destruction. This result would be caused 

 by the rain were it not that in most flowers the pollen is 

 well protected therefrom by its position, or other arrangements, 

 as will later be noted (page 295). A case of protective ad- 

 justment against excessive osmotic pressure seems involved 

 in the starch formation in leaves. In green leaves in the 

 light, as the student will recall, the appearance of starch is 

 always preceded by the formation of sugar, the starch being 

 formed only after a certain concentration of the sugar has 

 been reached. The starch, however, is always re-converted 

 to grape sugar when the concentration again falls, and thus 

 is translocated into the stem. Now this seemingly useless 

 formation of starch finds an explanation in the fact that while 

 grape sugar exerts osmotic pressure, starch exerts none. 

 The conditions are all consistent with the supposition that 



