DISPERSAL AND GERMINATION OF SEEDS 175 



advantage in being available for other purposes after the 

 seed has germinated and the young plant (seedling) has 

 begun to manufacture its own food. The " suitable tem- 

 perature " explains why seeds are not planted out of doors 

 until the " ground has warmed up " a little. It is well known 

 that plants vary widely in the amount of heat that is " suit- 

 able " for the germination of their seeds. For example, some 

 seeds germinate in early spring or even on the melting snow- 

 fields of alpine and arctic regions, while others need tropical 

 heat. The supply of oxygen necessary comes in the air, so 

 that there must be free access of air. This explains why it 

 is necessary to have the soil loose, so that there may be a 

 free " circulation of air " through it. If the soil is flooded 

 with water, so that there are no free passageways for air, 

 seed-germination is interfered with in the same way that an 

 air-breathing animal is interfered with if put under water. 

 If these three conditions are supplied, and the seed is really 

 a good one, it will germinate. 



105. The entrance of water. When a seed has been 

 placed in the proper conditions for germination, the first 

 visible result is its swelling on account of the entrance of 

 water. It must be remembered that the protoplasts of the 

 embryo and of the endosperm (if present) are in a dormant 

 stage, and this stage is associated with a lack of water, so 

 that the protoplasts are inactive (see 10, p. 11). The entrance 

 of water into the seed changes this dormant condition into an 

 active condition. Figuratively speaking, the embryo, which 

 has been " asleep," at least through one winter, " wakes up " 

 and resumes its activity. 



106. Respiration. The superficial indication that the 

 embryo has become active again is that oxygen enters the 

 seed and carbon dioxide escapes from it, a gas exchange that 

 indicates that respiration is going on (see 31, p. 38). In 

 other words, the embryo shows that it is alive and in an active 

 condition by " breathing," which is a test of life that we apply 



