216 The Phenomena of Morphogenesis 



One is the general phenomenon of heterophylly, where two or more 

 widely different types of leaves, usually without intermediate forms, may 

 occur on the same plant. This difference is most commonly, though not 

 always, associated with the plant's ability to live either submersed in 

 water or rooted in the ground with its shoots in the air ( Gliick, 1924, and 

 p. 330 ) . This may be interpreted as a case of heteroblastic development in 

 which the manifestations are reversible. It is related to the problems of 

 juvenile stages and progressive development discussed in the preceding 

 section. 



In many species of the pondweeds (Potamogeton) the floating leaves, 

 which rest on the surface of the water, are relatively broad and have an 

 internal structure not unlike ordinary herbaceous foliage, whereas the 

 leaves borne under water are long, narrow, and membranous, thus being 

 adapted to live as submerged organs. Somewhat similar differences may 

 be seen in various "amphibious" plants (p. 332), such as the water butter- 

 cup (Ranunculus aquatilis) and the mermaid weed (Proserpinaca palus- 

 tris ) . These species live in environments where part of their foliage grows 

 in air and part is submersed under water. Under the former condition, 

 the leaves are relatively broad and well provided with stomata and inter- 

 cellular air chambers. In the latter they are much dissected and thinner. 

 These effects of the environment on water buttercup were observed by 

 Lamarck and played an important part in the development of his theory 

 of evolution. The relation of differences between the "water" and "land" 

 forms in such plants to those between juvenile and adult stages has been 

 discussed by various workers. Burns (1904) believes that the "water" 

 form of Proserpinaca is the juvenile stage, associated with unfavorable 

 conditions, and the "land" form the adult type and associated with flower- 

 ing. He found that only the broad, entire leaves were formed in the 

 flowering season and only the dissected ones in the winter. Whether all 

 such cases of heterophylly may thus be interpreted is a question. Vischer 

 ( 1915 ) has called attention to the close relation between certain environ- 

 mental factors (such as removal of leaves, weak light, damp air, deple- 

 tion of carbohydrate reserves, and increased soil fertility ) and the produc- 

 tion of juvenile foliage (see also p. 206). Factors which favor reproduction 

 tend to produce adult foliage. Under certain conditions a return to the 

 juvenile condition may be induced (Woltereck, 1928). There is no evi- 

 dence that the ribbon-like submersed leaves of Potamogeton are juvenile 

 in type, however. Arber (1919) points out that in Sagittaria (another 

 "amphibious" plant) the first leaves are thin and ribbon-like even when 

 the plant is growing out of the water and that they appear at maturity 

 whenever the plant grows weakly. She believes that an aquatic environ- 

 ment is not responsible for heterophylly but that the occurrence of hetero- 



