CONDITIONS ESSENTIAL TO GROWTH 43 
it will swell up as a living seed would, but it will not send 
out a stem and root ; in other words it will not grow. 
(2) A supply of water is necessary. If a few seeds 
are placed in dry flannel—and some others in moist flannel 
—the former will remain unchanged whilst the latter will 
swell up, burst, and begin to sprout. Water is required 
because (a) it forms part of the living substance of the 
plant ; (b) it is a food substance, it is in fact plant-food ; 
(c) it is the vehicle by means of which the food-stuffs are 
carried to the growing parts; (d) it keeps the soft green 
parts stiff and extended, a condition essential to growth. 
(This was referred to on page 29, dealing with transpiration). 
(3) Free oxygen is required. If the air is freed from 
oxygen, growth becomes impossible. To show that this is 
so, take two bottles, with closély fitting 
corks, and from the under surface of each 
cork (which must be air-tight) suspend a 
small piece of sponge in the pores of which 
some seed such as cress is sown. Before 
suspending the sponges, pour a little water 
= into one of the bottles, and some pyrogallic 
| acid into the other. In the sponge which 
Fe ees tied “hangs over the water the seedlings quickly 
over pyrogallic — -hegin to germinate. In that suspended 
over the pyrogallic acid, no germination 
takes place, for the acid has withdrawn all the oxygen from 
the air and so prevented growth. We can now understand 
why plants sometimes die in very moist ground. The soil 
becomes so saturated with moisture, that the air is driven 
out, and the roots deprived of oxygen actually die of 
suffocation. 
(4) The temperature must be suitable. lxperi- 
ments will readily suggest themselves to show that seeds or 
plants exposed to extremes of temperature will not grow. 
Fig. 36.—Sponge 
