PERIODICAL OPENING AND CLOSING OF FLOWERS. 15 



excitement of light. As the higher temperature of summer is 

 produced especially by the degree and duration of insolation, so 

 this in turn may be regarded as a function of the apparent daily 

 course of the sun. But if the duration of sleep, its limits, and 

 the times of the greatest expansion are immediately dependent on 

 insolation, the epochs relative to the sleep of plants in diffei'ent 

 species are only so far comparable with one another, as they are 

 referred to the same length of day. 



The following tables in connexion with this subject now 

 demand our attention : — 



Plants which wake from sleep before sunrise : 



Antkericuni ramosuui. 

 Ipomoea purpurea. 

 Catananche versicolor. 



Plants which wake before the beginning of insolation : 



Ipomcoa purpurea, 5 hours before or — 5 



2 species — 4 



7 „ —3 



9 „ — 2 



19 „ —1 



While then 4 per cent, of the plants examined open their 

 blossoms before sunrise, this is the case with 45 per cent, 

 before they are reached by the first direct sunbeam. As many 

 species, therefore, seem to require the direct light of the sun in 

 order to their expansion, as those which need only that which is 

 diffused in the atmosphere ; a very few only are so delicately 

 susceptible, as to expand at the glimmer of twilight. 



Plants which wake at sunrise : 6 species. 



Plants whose blossoms open at the commencement of insola- 

 tion : 22 species. 



It is clear that sunrise is only so far an important moment for 

 the physiognomy of the floral world, as the position in which 

 plants are placed is favourable to their immediate insolation, 

 which can only be the case where the field of view is perfectly 

 clear. Although the greater number of plants must be exposed 

 to the direct rays of the sun before their flowers expand, yet we 

 cannot suppose that this must necessarily take place at sunrise, 

 when the position is such that it is accessible to the rays of the 

 rising sun, and is not first insolated when the sun has attained a 

 high angle. It is more probable that the latter condition is 

 requisite, in order that the insolation may have attained a due 



