by Deciduous Plants in the T'ertiary Floras. 293 



Tertiary plants congeneric with those of modern Europe lost 

 their leaves, like the latter, at a certain period of the year, it is 

 evident that we must seek for this periodical fall an original de- 

 termining cause other than that of a diminution of temperature. 

 It is true that we may, and that we even must, assume the exist- 

 ence of a season, not cold, but fresher and moister, succeeding 

 to the hot season, reanimating vegetation instead of extinguish- 

 ing it, and bringing on the flowering of the plants — a season 

 rather of life than of sleep and death, and therefore very different 

 from our winter. What might be the effects of such a season 

 upon the plants which we believe to have had deciduous leaves 

 is the problem which is now before us ; but it is necessary, in 

 the very first place, to ascertain whether cold — that is to say, 

 the sinking of the thermometer below the degree of heat ne- 

 cessary to the vegetation of each species — is the actual cause 

 of the interruption of this vegetation during winter, or whether 

 this cold only serves to render this interruption longer, more 

 complete, and more radical, by coinciding with the period at 

 which it is naturally manifested. 



Now, when set in these terms, the question is easily solved. 

 It is evident that all trees suffer from thermometric cold. For 

 those of our continent this cold is a crisis which they pass 

 through at a moment when their organs are in a condition to 

 offer it the most resistance. The sleep in which they are sunk, 

 at the same time that it favours the internal elaboration of their 

 organs, allows them to undergo the crisis of cold without incon- 

 venience ; but this crisis, it must be said, is neither the reason 

 for the existence nor the true cause of their physiological con- 

 dition, as may easily be proved. We may, in fact, lay down as 

 a principle, that, in the very great majority of cases, the trees 

 with deciduous leaves lose their leaves at a higher temperature 

 than that which subsequently causes the evolution of new leaves. 

 This phenomenon may be easily proved in southern countries, 

 and even in Provence; it becomes very striking in the hot 

 regions which permit the growth of tropical organisms side by 

 side with those which are peculiar to the northern parts of our 

 hemisphere. In Madeira, for example*, where, in consequence 

 of a very great uniformity of temperature, there is scarcely any 

 winter, the vegetation, taken in its totality, is never interrupted. 

 A multitude of plants, and especially the Laurine^e, Myrtacece, 

 Passijiora, Biynoniacea, &c., both indigenous and exotic, blossom 

 during this season, which is that in which the gardens present 

 the most ravishing spectacle. Nevertheless this continuous 

 mildness of the temperature is no obstacle to the progress of 



* This observation is due to M. Heer, who resided for a considerable 

 time in Madeii'a. 



