24 SEEDS AND SEEDLINGS. 



cent.). This amount of water, though not sufficient to allow of ger- 

 mination taking place, is evidently necessary for the seed to remain alive 

 and capable of germinating. The weight found after thorough drying, 

 without charring, is called the dry-weight of the seed or seedling. 



* To dry seeds, seedlings, leaves, etc., without charring them, use a 

 slow oven, a water-bath, or a sand-bath over a small flame. A simple 

 sand-bath consists of a shallow tin or pan filled with sand, supported 

 on a tripod and heated below as usual, the seeds being placed in a 

 smaller tin or a saucer resting on the sand. 



46. Absorption of Water by Seeds. In the absorption 

 or imbibition of water by dry seeds we have an interesting 

 subject for investigation. Imbibition is due to the attraction 

 of the particles (molecules) of the two substances for each 

 other, which partly overcomes the cohesion of the molecules 

 of the imbibing substance, making them separate so that the 

 substance swells. It makes the tissues more porous by 

 separating the molecules and is thus an important aid to 

 osmosis (Art. 51) the process which controls the diffusion 

 of liquids through membranes. Imbibition is increased by 

 warmth and by the presence of dilute acids and alkalis, but 

 is diminished by that of salts. The work done in imbibition 

 is shown by the force exerted by swelling seeds or wood, and 

 by the fact that heat is given off. 



(a) Keep some "dry" seeds in a drying-oven or drying-bath until 

 they show no further loss in weight, and then find out whether the}' 

 swell up in water arid whether they germinate. The results will 

 show that killed seeds still have the property of absorbing water. 



* (6) When a dry seed is placed in water, how much does it absorb, 

 and what proportion do the volume and weight of the absorbed water 

 bear to the volume of the dry seed ? Weigh twenty dry Beans ; pour 

 water into a graduated vessel until it reaches the 150 c.c. mark, then 

 drop in the beans, and shake the vessel to get rid of any air present ; 

 the rise in level gives the volume of the Beans. Take them out and 

 place them in moist sawdust for two days, then wipe them dry, weigh 

 them, and find their volume as before. If you have no graduated 

 vessels, use a glass jar with a strip of paper, marked into inches or 

 centimetres, gummed on the outside of the jar. Beans absorb about 

 130 per cent, of their own dry weight of water. 



* (c) The swelling of seeds by imbibition of water can be easily demon- 

 strated to a class. Put about 30 grammes of dry Peas and an equal 



* See footnote on p. 23. 



