NATURE OF PLANTS 129 



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 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. Light only in rare instances appears to be a contributing 

 factor in the renewal of growth. Some seeds will sprout at near 

 the freezing point, while others require a temperature between 

 10° and 17° C. It is interesting to note that the rising tempera- 

 ture together with other factors as the season advances from 

 early spring bring 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 con- 

 dition. Seeds soaked in water cannot endure such extremes as in 

 a dry state. Excess of water, however, 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 of 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 co-operate in 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 conditions 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- 

 pyle, exists in the integument. This opening is seen near the 

 scar or hilum that marks the point where the seed was attached 

 by a minute stem to the walls of the ovary (Fig. 83, B). This 

 opening permits the entrance of the air and water as does the 

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