274 



THE AMERICAN NATURALIST 



[Vol. LIII 



part of the plant, to the medium. The work of the last thirty- 

 years has, however, rendered this simple eoneeption untenable ; 

 the theory that now holds the field accords a much less prominent 

 place to adaptation. The first observation that shook the founda- 

 tions of the idea that leaf form necessarily depended directly on 

 the milieu, was that of Costantin, who showed that, in the ease of 

 Sagittaria, the aquatic and aerial leaves were already differen- 

 tiated from one another in the submcrired bud; he noticed au- 

 ricles on a leaf which was only 2 to 3 mm. long. In Eanunculus 

 heteroplnjllus, also, the leaves destined to be aerial are differen- 

 tiated in the bud. 



A large amount of experimental work has been published by 

 various authors on the effect of conditions upon the leaf forms of 

 heterophyllous plants, and, although some of the results are con- 

 fused and conflicting, a study of the literature seems to justify 

 one general conclusion — namely, that, in many cases, the sub- 

 merged type of leaf is, in reality, the juvenile form, but can be 

 produced later in the life history in consequence of poor condi- 

 tions of nutrition ; the air 'leaf, on the other hand, is the product 

 of the plant in full vigor and maturity. This conclusion, which 

 is primarily due to Goebel and his pupils, is substantiated not 

 only by experiments but by observations in the field. 



In many heterophyllovis phiiits, the first leaves produced by 

 a seedling, whether it (i('vcl(>i)s on liind or in water, conform, 



stance, in the Alismacea'. In Ahsnni jihuihir/o. the water i)lantain, 

 and Sagittaria sagittifoJia, the arrowhead, the first leaves pro- 

 duced by the seedling (or the germinating tuber) are ribbon- 

 like, even when the young plant is terrestrial. The formation of 

 this type of leaf can be induced again, even in maturity, by con- 

 ditions which cause a general weakening of the plant. Costantin 

 thirty years ago, recorded that, when the leaves of Alisma plan- 

 tago were cut off in the process of clearing out a water course, or 

 in a laboratory experiment, the next leaves produced were rib- 

 bon-like, thus representing a regression to the submerged form. 

 More recently, another worker, Waehter, tried the experiment of 

 cutting off the roots of healthy, terrestrial plants of Sagittaria 

 natans bearing leaves with differentiated lamina?. It was neces- 

 sary to cut the roots away every week, as they grew again so 

 ra,|)idly. The result of this treatment was that the plants were 

 found to revert to the juvenile stage, the new leaves being band 

 shaped. When the experimenter ceased to interfere with the 

 roots, the plants again formed leaves with laminfe. Other plants, 



