2IO 



PLANT PHYSIOLOGY 



of water. In this connection, however, the student should ac- 

 quaint himself from his books with our present knowledge of 

 the mechanical relations of -water to growth, especially the rdle 

 of osmotic pressure in forcing the enlargement of cells. 



GERMINATION OF SEEDS FOR EXPERIMENTAL PURPOSES. This is a phase 

 of growth having a very practical experimental application. Where it is neces- 

 sary to obtain roots and their hairs in a condition as perfect as possible, the 

 best arrangement is the saucer germinator, shown by figure 57. It consists 



FIG. 57. SAUCER GERMINATOR; Xi- 

 Made of four porous saucers, as explained in text. 



of a cleaned, soaked, and preferably sterilized, flower-pot porous saucer 

 holding the seeds (which are previously soaked), and covered by another 

 of the same sort. This is placed inside a larger covered saucer which is 

 supplied daily with water to a depth somewhat greater than the thickness 

 of the bottom of the seed saucer. Thus the seeds are supplied with a suf- 

 ficiency of water which is filtered by the saucer. The air should be renewed 

 daily by lifting the covers for a moment, and blowing out the old air. This 

 forms for most purposes an ideal germinator, fully as good as a Zurich or 

 other specially constructed germinator. It has, however, for some pur- 

 poses the drawback that the roots do not grow straight, nor is it useful where 

 a considerable growth of stem also is needed. For this, and for growing seed- 

 lings generally, the very best arrangement is a flower-pot or box filled with 

 sphagnum moss, moderately packed; this material forms the best-known 

 medium for the purpose, as it combines almost ideal aeration and moisture, 

 and has little tendency to develop mould. Even the little moulding it does 

 develop can be prevented by occasional sterilization with- steam, and it is 

 advantageous to have in the laboratory a jar of sterilized moss, with another 

 jar for that which has been used. The moss is so efficient that it is worth 

 while to take some trouble to obtain it; it is sold by all dealers in gardeners' 

 supplies, and may be used repeatedly. Next in value is sawdust, of which 

 that made from pine, and hence free from tannin, is said to be best. In 

 general the best depth to plant seeds is about three times their own (least) 

 diameter, and they grow a little faster if the radicle end is pointed down- 



