156 HETEROPHYLLY [ch. 



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 

 confused and conflicting, a study of the literature seems to 

 justify one general conclusion namely, that, in many cases, 

 the submerged type of leaf is, in reality, the juvenile form, but 

 can be produced later in the life-history in consequence of poor 

 conditions of nutrition ; the air leaf, on the other hand, is the 

 product of the plant in full vigour 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 heterophyllous plants, the first leaves produced by a 

 seedling, whether it develops on land or in water, conform, more 

 or less, to the submerged type. This is the case for instance in 

 the Alismaceae. \n Alisma Plantago (Fig. loi A 2^x6. 5, p. 153) 

 and Sagittaria sagittijolia (Fig. 90, p. 141), the first leaves 

 produced 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 

 conditions which cause a general weakening of the plant. 

 Costantin^, thirty-four years ago, recorded that, when the leaves 

 of Alisma Plantago were cut off in the process of clearing out 

 a water-course, or in a laboratory experiment, the next leaves 

 produced were ribbon-like, thus representing a regression to 

 the submerged form. More recently, another worker^ tried 

 the experiment of cutting off the roots of healthy, terrestrial 

 plants of Sagittaria natans which bore leaves with differentiated 

 laminae; it was necessary to cut the roots away every week, as 

 they grew again so rapidly. 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 

 laminae. Other plants, with uninjured roots, grown as water 

 cultures in distilled water, also produced the juvenile leaf form, 



1 Goebel, K. (1896), etc. 2 Costantin, J. (1886) 



3 Wachter, W. (1897I). 



