CONDITIONS OF GERMINATION 433 



necessity of burial. Peculiar modes of germination in other 

 palms are described by Cook (19 12). 



Temperature affects the rate of water entry into the seed 

 and the growth rate, the latter much more than the former. 

 Temperature may also affect the resistance offered by the 

 seed coats to the extrusion of the radicle. Many deter- 

 minations have been made of the " minimum," " maximum," 

 and " optimum " temperatures for germination. Thus it 

 has been found that the minimum temperature at which rye 

 will germinate is about 1° C, the maximum 30° C, and the 

 optimum 25° C. For barley the figures are 3° C, 28° C, 

 and 20° C, and for maize 8° C, 44° C, and 32° to 35° C. 

 The minimum points are of some interest in showing at 

 what temperature the seed will lie dormant in the soil 

 without the action of any other factor. They mean that 

 germination can take place rapidly only at considerably 

 higher temperatures. As Blackman (1905) has pointed 

 out, " optimum " points are purely fictitious, varying 

 especially with the time through which the factor in question 

 has operated. This comes out very clearly in the case of 

 germination, where the temperature at which the radicle 

 appears most quickly may be so high as to inhibit or depress 

 further growth ; that is, for a short time the high temperature 

 causes rapid growth, which quickly declines owing to the 

 action of the " time factor." With some seed, e.g. the barley, 

 and Petunia, a daily alternation of high and low temperatures 

 is required to give good germination (Harrington, 192 1) ; 

 as we have seen, this may be connected with alterations in 

 the seed coats. 



§ 5. Germination — Liberation of the Embryo 



The emergence of the radicle from the seed takes place 

 against the resistance of the enclosing walls, the seed coats, 

 or, in indehiscent fruits, the fruit walls. This resistance may 

 be quite small, as in thin-coated seeds like the pea, or it may 

 be large, as in the castor-oil bean with its hard testa . FamiHar 

 examples of strong walls are such seeds as those of the 



2 F 



