630 GERMINATION AND DORMANCY 



remains within the seed and acts as an absorbing organ through which soluble 

 foods in the endosperm move into the tissues of the rapidly enlarging embryo. 

 Soon after the appearance of the primary root the plumule and the colcoptile, 

 which completely encloses it, grow out through the walls of the grain and 

 upward as a result of elongation of the region of the axis just below the 

 plumule. About the time the coleoptile breaks through the surface of the 

 soil, or soon thereafter, the first foliage leaf grows through the tip of the 

 coleoptile and emerges into the light and air. 



Environmental Conditions Necessary for Germination. — The seeds of 

 all species of plants require at least three external conditions before germina- 

 tion can occur: (i) water, (2) a suitable temperature, and (3) oxygen. A 

 fourth factor, light, appears to be essential for the germination of the seeds 

 of a few species and to influence the germination of seeds of some other species. 



1. ITater. — A low water content is one of the prominent characteristics 

 of resting seeds and since physiological processes occur largely in an aqueous 

 medium, germination cannot occur unless the seed can absorb water from 

 its environment. The absorption of water by seeds initiates the series of 

 physical and chemical processes which, in the absence of any limiting factor, 

 results in the emergence of the embryo from the seed. In general, however, 

 complete germination of seeds will not occur in a soil with a water content 

 below the wilting percentage (Chap. XVI). 



Water-vapor as well as liquid water can be imbibed by seeds. Most seeds 

 will therefore pass through the earlier stages of germination in an atmosphere 

 which is saturated, or nearly so, but if the vapor pressure of the atmosphere 

 is appreciably below the saturation value germination will be checked or 

 inhibited. 



2. Oxygen. — The respiration of germinating seeds proceeds at a rapid 

 rate especially during the early stages of germination. The partial pressure 

 of oxygen in the atmosphere can be considerably reduced, however, without 

 greatly interfering with the rate of respiration (Chap. XXIX). In fact the 

 seeds of some water plants such as cat-tail {Typha latifolia) germinate 

 better under low oxygen pressures than in air. Seeds of many terrestrial 

 plants can germinate under water where the concentration of oxygen often 

 corresponds to a partial pressure of oxygen very much less than that of the 

 atmosphere (Morinaga, 1926). It is probable that seeds buried deeply in 

 compact soils are often prevented from germination by the very low partial 

 pressure of oxygen in such an environment. During the early stages of germi- 

 nation of seeds of pea and some other species respiration is largely or almost 

 entirely of the anaerobic type because of the relative impermeability of even 

 the hydrated seed coats of such species to oxygen (Chap. XXX). As soon as 



