127 



association, of which it is a characteristic species, takes place. 

 This would seem to indicate that such forms are responses to 

 environment. Consequently their distinguishing characteristics 

 are not characters of organization. If this were not so, such 

 forms would hold valid claims to specific rank. Such modifica- 

 tions occur constantly, but only occasionally are they of impor- 

 tant ecological significance. It may happen to several, and 

 sometimes to all, of the species growing in a certain habitat. 

 There need be no taxonomic relationship between the species so 

 involved. 



The modifications most frequently observed tend towards the 

 conservation of water supply. These are observed on soils made 

 up chiefly of sand and gravel. The plants themselves are usually 

 smaller. They are frequently more pubescent than usual. The 

 leaves are narrower, thicker, often rolled, and frequently assume 

 positions of protection from the noonday sun. The root system 

 is more extensively developed, the flowers and fruit, however, do 

 not ordinarily exhibit noticeable differences from the ordinary 

 type. There is frequently a tendency to bloom more freely un- 

 less the growing conditions are extremely severe. 



Petalostemum purpureum f. arenarium forma nova* 





Petalostemum purpureum 

 (Prairie plant) 



Petalostemum purpureum f . arenarium 



Root 



tap root 



larger and more bulky tap root 



Crown 



composed of a few upright 

 stems 



composed of many (20-38) radiating stems 



Stems 



stout and upright 



shorter, wiry, divaricate, /. e., standing at 

 an angle of less than 45° with the earth 

 from the commencement of growth. 

 When growing on little hillocks the stems 

 project below the horizontal 



Leaves 



divaricate, lancolate-trifoli- 

 olate 



appressed, linear-trifoHolate 



Heads 



cylindrical, larger 



cylindrical, smaller relatively 



Flowers 

 and Fruit 



no app 



reciable differences 



An ecologist meets with such a state of affairs quite frequently, 

 and these extremely varying forms may occasionally be of such 



* Planta caule procumbente ab initione, foliolis lineariis, arenariam incolat. 



