428 Minnesota Plant Life. 



plant, an instance for which no doubt other and special reasons 

 must be assigned. In general the so-called sleep of leaves and 

 flowers is an adajitation to the falling' temjjerature after the sun 

 has set. 



\\nien plants grow in warm regions they often manifest a 

 number of characters which are the reverse of those to be 

 looked for in j^lants whose home is nearer the poles, or at a 

 greater elevation. Thus in many tropical plants the produc- 

 tion of special reserve storage organs is less considerable than 

 among related species of colder climes. Buds are not so care- 

 fully ])rotected. Furry coats on the leaves or twigs are not so 

 abundantly formed — except, it should be said, by desert plants 

 in which furry or hairy coatings arise as a protection against 

 the too ardent rays of the sun. In the tropics, since hiberna- 

 tion is unnecessary, the various hibernating habits fail to ap- 

 pear, and plants do not so commonly grow as biennials. The 

 foliage is not shed at the end of a definite growing season, but 

 normally drops only when exhausted or when shaded out of 

 existence by younger foliage between it and the sun. The 

 heat-producing qualities, as manifested by color, are not so 

 marked a feature, and a great many white, yellow and mottled 

 flowers occur, like those of most tree-top orchids in the ecpia- 

 torial forests. Evergreen plants, such as the live-oak, flourish 

 in the south; in tem])erate regions the oaks arc deciduous, 

 though retaining the character of trees, while in the far north, 

 oaks, which would be trees in more southern ranges, become 

 stunted into little shrubs. It is possible, in \iew of such facts, 

 to understand how dwarf shrubs may occur in certain genera, 

 as, for example, the dwarf cornel among the dogwoods; the 

 dwarf snowberry, among the honeysuckles ; or llic dw arf willow, 

 among the willows. It is ])ossible to understand, too. how very 

 hairy varieties ma_\- (k'\elop under climatic conditions different 

 from those which faxor the ])ro(luclion of smooth \arieties of 

 the same plant. The silky dogwood may thus be regarded as 

 a species showing in its foliage a response, either to the lower 

 average temperature, or to more direct illumination by the sun 

 than the smooth-leafed dogwood. 



Light. Another form of energy, which has a strong effect 

 upon the structure and habits of plants, is light. Some plants are 



