THE PLANT WOELD 171 



A DEQDUOUS TROPICAL TREE. 



By O. F. Cook. 



THAT the trees of temperate regions lose their leaves in the winter, 

 while those of the tropics are evergreen, affords one of the most 

 striking contrasts between the two zones. The tropics with their 

 perennial verdure are commonly thought of as the headquarters of the 

 vegetable kingdom, since the temperate regions are open only to the 

 species able to withstand frost. With many herbaceous plants only the 

 seeds are resistent, in others the underground pai-ts are protected by 

 the soil. Still other temperate herbs are dependent upon a covering of 

 snow. Shrubs and trees, however, must endure all climatic extremes. 

 At the northern limit of the range of a species great advantage is en- 

 joyed hj a tree more hardy than its fellows, and the deciduous habit 

 has accordingly been interpreted as an indubitable instance of a char- 

 acteristic attained by natural selection, that is, our temperate trees are 

 held to have become deciduous because of frosts which killed all non- 

 resistent individuals. Interpreted into a general proposition, this is 

 taken to mean that changes in the characters of organisms are due to 

 their environment. 



Darwin and the earlier evolutionists were accustomed to think of 

 variations as being extremely small or almost imperceptible ; but we now 

 know that they are very frequently considerable, so that the line of ad- 

 vantage or disadvantage may easily be crossed in a single generation. 

 In other words, plants or animals move of themselves much more act- 

 ively than has been supposed under the theory tliat such changes took 

 place only when imperceptible differences were accumulated through 

 the close sifting of natural selection. Too great confidence in the sup- 

 position that the deciduous habit has been attained only through selec- 

 tion in temperate regions has led many to the belief that the relatively 

 few deciduous trees of the tropics are descended from temperate ances- 

 tors, and it is indeed a most striking reminiscence of our northern 

 autumnal season to see a giant sillc-cotton or other Bombacaceous tree 

 standing leafless in a full-foliaged tropical forest. It has been supposed 

 that the time passed in this inactive condition is wasted, and that such 

 a disadvantageous habit can be explained only on climatic grounds. 

 The interruption of growth is not, however, confined to temperate re- 

 gions nor to deciduous trees. In fact, the vast majority of plants and 

 animals, tropical as well as temperate, have alternating periods of rest 

 and vigorous growth. The falling of the old crop of leaves may be a 

 mere incidental of such a process. Among the numerous native species 

 of figs of the interior of Guatemala some are completely deciduous in 

 the dry season, while in others the full vigor of the leaves is retained. 



