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1902] 



CHANGE OF FORM IN PROSERPINACA PALUSTRIS 



101 



while the latter developed the air form. Jn general, the optimum 

 temperature for the two forms, so far as could be observed, is 

 about the same. This indicates that temperature, in itself, is not 

 a factor in determining" which form shall be produced. 



4. Salts. — The relations of carbon dioxide and oxygen to a 



1*IG, 8. — Plants after being submerged two months. 



possible stimulus will be discussed later under another head, as 

 well as the question of food salts. Plants were grown in distilled 



in tap water, and in nutrient solutions. Some were 



water 



entirely submerged, with their roots in the soil ; others had the 

 soil washed away from the roots, and shoots and roots submerged 

 in the water. Some in both distilled and tap water were sub- 

 merged except the very tips, which were in the air. The same 

 was done in various strengths of nutrient solution. All of these 

 were repeated in strong and weak illumination. The results were 

 invariably the same. The finely divided leaf, with the aquatic 

 type of structure in the leaf and shoot, always occurred when the 

 growing tips were submerged, it mattered not whether in dis- 

 iHed water or in nutrient solutions, or in strong or weak illumi- 



