26 



MISSOURI BOTANICAL GARDEN. 



that the artificial arrangement of pools, ponds and streams 

 brings within a small space a varied number of natural con- 

 ditions such as small bodies of quiet, shallow, constantly re- 

 newed water with or without outlet, swiftly and slowly run- 

 ning streams, etc. 



So far as the sources of a fresh-water algal flora are con- 

 cerned, there are present the possibilities of dissemination by 



(1) currents of air bearing spores through short distances; 



(2) transportation by birds, ai 



lm 



insects. It 



been observed in a study of the dissemination of Lemna, by 

 Mr. C. II. Thompson, th&tBelostoma americanum, commonly 

 found flying about electric light globes on the street, carried 

 Lemna attached to its body. This insect stays in the water 

 during the day and flies about at night, If this is true of 

 Lemna it might easily be true of algae which grow in the 

 same habitats. 



m 





(3) Introduced plants in ponds and pools, 

 many of which are of tropical origin, thus having been de- 



3ly differing habitats. There is but slight pos- 

 sibility of dissemination through the agency of water cur- 

 rents directly except within the garden itself, for only two 

 small streams enter it, and the boundaries due to street or 

 grading are higher than the surrounding territory. Of the 

 two small streams, which enter, the one from the south drains 

 the grassy, sparsely wooded portion of Tower Grove Park and 

 during the period under observation has been quite free from 

 algal growth large enough to be noted without microscopic 

 inspection. The stream which comes from an ordinary city 

 block on the west contains few or no algae and is often dry. 

 It is evident from the chart that the main bodies of water 

 within the garden are connected and it is to be noted that 

 the city water at its entrance is free from all vegetation. 



Having considered these deviations from the probable 

 typical environmental characters and those characters bear- 

 ing on the source of the algal flora, it is evident that this 

 large number of forms present, offers a good opportunity for 

 consideration of their relative grouping and adaptation to 

 habitat conditions which in range and variety express in 



miniature the probabilities of a much larger area. 





