46 THE AMERICAN BOTANIST 



that in the fall they should be called simply Helianthus oc- 

 cidentalis. It seems, then, that there are botanists who hold 

 that a species and a sub-species may grow from the same root 

 and on the same stem, at different seasons ; may, indeed, gradu- 

 ally pass from one category to the other as the year rolls by. 

 What boundless possibilities do such conceptions open up. 

 What a multitude of plants must be provided with sub-species. 

 The case is especially strong for those trees which have very 

 different juvenile and adult foliage. In fact, why is not tl.e 

 bean of early spring showing only its cotyledons a good sub- 

 species of the very different plant which later in the season, 

 under the stimulus of nutriment moisture and light succeeds it 

 at the time of blooming and fruiting? 



Aggressive Plants. — It seems to me that we too often 

 err on the side of making phenomena more simple than they 

 really are. Plants are vastly more complex organisms than 

 our formulated ideas recognize. Many of their phenomena 

 completely baffle us. For example, I might mention what has 

 been called aggressiveness in a plant namely, its ability not 

 only to occupy and maintain the soil but to spread and crowd 

 out other plants. This is particularly evident in plants intro- 

 duced from one country into another. Thus nearly all our 

 wee'ds are of Old World origin. The same is true of our 

 permanent meadow and pasture plants, where ability to oc- 

 cupy and hold the ground against weeds is essential. In this 

 respect our American grasses and clovers utterly fail before 

 the foreign immigrant. Some other striking instances of the 

 great aggressiveness of an immigrant may be cited. The in- 

 troduced English violet is said to be one of the worst weeds in 

 Mauritius; American cacti are becoming a pest in South Af- 

 rica; the marvelous vigor and spread of the American water 

 weed {Elodea) under European conditions is well known. 



