NATURE OF PLANTS 115 



slowly formed during the resting period and a renewal of growth 

 is not possible until they have begun the transformation of the 

 insoluble foods into soluble forms. The changes thus effected 

 are so considerable that growing tubers, root stocks and seeds 

 often have a sweet taste owing to the considerable amount of 

 sugar present. So the seed may be looked upon as a variation 

 in the growth of the plant that enables it to multiply and dis- 

 tribute its kind and to meet the various problems in connection 

 with its existence, such as drought, cold, and sometimes even 

 fires. 



56. Conditions Necessary for Growth. — The external condi- 

 tions necessary for starting the growth of the embryo are a 

 suitable temperature, moisture, and the oxygen of the atmos- 

 phere. Some seeds will germinate at near the freezing point, 

 while others require a temperature between io° and 17° C. It 

 is interesting to note that the rising temperature as the season 

 advances from early spring brings out a regular succession in 

 the awakening of various kinds of seeds. A suitable amount 

 of moisture must also be supplied to the seed to assist in the 

 solution and diffusion of the foods to the embryo. This is 

 very necessary since the seed contains very little water. In fact 

 the seed owes its vitality and power to resist long drought and 

 severe cold to its dry condition. Seeds soaked in water cannot 

 endure such extremes as in a dry state. Excess of water, how- 

 ever, is unfavorable to the renewal of growth, because it excludes 

 the oxygen of the air. This is the cause of the failure to grow 

 and the decay of the seeds planted in the spring when the season 

 is rainy. The air spaces in the soil become filled with water 

 and as a consequence oxygen can not gain access to the seed 

 and start those chemical changes that not only supply some of 

 the foods to the embryo but also provide it with the energy for 

 growth. An examination of a few seeds will show some of the 

 more important structural features that adapt them to the con- 

 ditions that they have to meet. 



In the case of the bean the integument is tough and would 

 appear to offer a rather effectual barrier to the entrance of water 

 and air. You have noted that a minute opening, the micro- 



