262 OHIO STATE ACADEMY OF SCIENCE. 



there is coupled with the small surface exposed a prostrate habit 

 which shelters them from the wind and allows them any ad- 

 vantage to be gained from warmth radiated from the ground. 

 In great contrast to these dwarf creeping almost herbaceous 

 forms are the tree willows so well known to every one. These, 

 by their rapid growth are enabled to succeed well in their 

 favorite habitat along streams and lakes because they can 

 quickly repair the damage done by floods and storms, while 

 more slowly growing trees would be almost hopelessly beaten 

 down and destroyed before they could rear their trunks to a 

 sufficient size to enable them to withstand the elements. Like- 

 wise their well known ability to grow from cuttings and broken 

 branches renders them able to use the very storms which break 

 them in pieces as the means of their further dispersal. 



Their well known variability under different conditions is 

 an evidence of plasticity of constitution and adaptability to 

 various environments which is as important a factor in the success 

 of a plant in meeting the competition to which it is subjected as 

 is the same quality in the success of a man. From their adapta- 

 bility, variability and the large number of intermediates between 

 very divergent forms we may safely infer that we have here a 

 group of species with a very large amount of "Vital Motion" in 

 rapid course of evolution. 



ON USING THE KEYS. 



To construct a key which will hold for all individual willows 

 is well nigh impossible, because in the first place, there is a greater 

 or less number of hybrids for which no pretense at a key is made ; 

 and secondly, there is so much variation in some of the species 

 that even after including them in two or three different places, 

 the writer is not so sanguine as to suppose that he has covered 

 all the variant forms. Bearing this probable defect of the key 

 in mind, it will be understood that the greater the diligence with 

 which each clue given is followed out, the greater will be the 

 probability of a correct determination. If the plant you have 

 seems to fit in both divisions of the key by all means run it 

 through both and after doing so refer to the descriptions and 

 plates as well. 



The synoptic table is intended primarily to give some idea 

 of the relationships of the different species and consequently the 

 whole anatomy of the plant is used while all the parts are never 

 present in a single specimen. But in the keys proper every- 

 thing is subordinated to the end in view, namely the identifica- 

 tion of the plant ; and only such characters are used as are actually 

 present on a single specimen. In the foot-notes under the keys 

 their especial uses and limitations are discussed. 



