ADJUSTMENT TO TEMPERATURE 



129 



than during a warm one, and the flowers consequently appear 

 hiter. This sum is likewise obtained later as one goes northward 

 or ascends mountain ranges, and the time of flowering is cor- 

 respondingly delayed. The period during which a species remains in 

 flower is similarly though less evidently dependent upon temperature. 

 In the vast majority of species the period of flowering is largely 

 a fixed habit. This is especially evident in many annuals, and 

 in species that flower early in the spring. In nearly all cases its 



Fig. 35. Ephemeral night-blooming flowers of an evening primrose, Pachy- 

 lophus hirsutus. The flowers open at sundown; they close at 7-8 a.m. 

 the next day and quickly wither. 



position and length have been determined by the necessity of 

 rendering it possible for the seeds to mature before the time of 

 killing frosts. Species with relatively small and simple fruits, 

 such as the grasses and composites, may flower late, while those 

 with large or complex fruits, many roses, legumes, etc., usually 

 flower much earlier. 



Since flowering is so intimately connected with temperature, 

 flowers may be most conveniently classified in accordance with the 

 season in which they appear, or in which the period of flowering 



