338 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



In a discussion of Pond's experiments by Rcighard and Pond 

 (Pond, 1903), these authors conclude that, if Pond's observations are 

 correct, submerged aquatics when rooted not only act as a mechan- 

 ical support for algae, but also play an important nutritive role by 

 taking mineral food from the soil and organizing it into vegetable 

 matter. "Upon the decay of the vegetable matter this food material 

 is believed to pass into solution in the water. It should there nourish 

 the plankton algae." Rcighard and Pond ascribe Kofoid's results 

 to the fact that the vegetation with which ho was dealing was largely 

 Ceratophyllum, and therefore not rooted. 



In the experiments mentioned above in which PMlotria cama- 

 dends grew as well when floating free in the water as when rooted 

 in the soil, root hairs were not developed by the floating plants. This 

 suggests that Philotria draws a large part of its nutriment through 

 its leaves and may therefore compete with the plankton algae for 

 mineral salts, as well as for C0 2 . 



We have already seen that plants of Philotria grow better when 

 some C0 2 , other than that obtained by the water from the air, is sup- 

 plied them. The same thing is true of many algae. A large number 

 of these were tried in different solutions, both with and without the 

 artificial addition of C0 2 . In every case they made a much better 

 growth when C0 2 was added to the solution, and in a number of cases 

 in which they would not grow in a solution without the addition of C0 2 

 they made a vigorous growth when this was added. The need of an 

 abundant supply of C0 2 , which is shown by both Philotria and many 

 algae, would suggest that there may be a sharp competition for C0 2 

 between these forms, and that the growth of such plants as Philotria 

 would be detrimental to the production of phytoplankton. In a large 

 number of cases in which various algae were grown in jars with and 

 without Philotria, it was found that in every case, the algae grew much 

 better when there was no Philotria present. This was true in all of 

 the solutions tried, both when the Philotria was and was not rooted. 

 It is evident, therefore, that the presence of such a plant as Philotria 

 is in some way harmful to phytoplankton, and since variations in the 

 amount of C0 2 control the growth of the plankton algae to a great 

 extent it is not unlikely that the effect of Philotria on the algae is duo 

 in large part to the use of C0 2 by Philotria. It is also probable that 

 the amount of C0 2 present in a fresh-water lake will be a more import- 

 ant factor than the amount of mineral matter in determining the 

 quantity of phytoplankton produced. 



It has been shown that Great Lake with little submerged vegeta- 

 tion has a very scanty phytoplankton, while Lake Ellis, with a great 

 quantity of submerged vegetation, had an abundant phytoplankton. 

 These results appear at first to be contrary to the conclusions drawn 

 from the experiments; but in Lake Ellis the shallow water, allowing 

 the use of sunlight by the plants, is probably more favorable to both 



