ORIGIN OF EXDOOENS FROM EXOGEXS. 517 



as, e. g., between the endogenous Hydro charts and the exogenous 

 Limn ant hemum. 



We are indebted to Mr. W. P. Hiern for demonstrating the 

 truth of the above inference ; for he has shown by mathematical 

 calculations that the best form of the margin of floating leaves for 



«'"w 



resisting the strains due to running water is circular, or at least 

 the several portions of the margin wouid be circular arcs ; showing 

 that nature had adopted what was most serviceable in this respect 

 in such leaves as Ranunculus heterophyllus, NymphceacecE, Lim- 

 nantliemum, and Hydrocharis. 



Mr. Hiern concludes his paper with the following remarks: 

 " It is a tenable hypothesis, and by no means improbable, that 

 during much or most of the time when actual growth is taking 

 place and when the velocity of the current is subject to many 

 and various vicissitudes, the plant has the power of adapting 

 its growing efforts to the circumstances ju^t necessary for its 

 development " *. 



Embryonic Character of the Leaves of Exdooexs. — That 



the prevailing ribbon-like form of submerged leaves of eudogens 

 is of a more u embryonic " character than the dissected type of 

 submerged leaves of exogens, would seem to be shown by those 

 of Hippuris and Callitriche. The leaves of those plants, in fact, 

 resemble cotyledons. Callitriche has three ribs radiating from 

 the base, while Ilippuris has a still simpler leaf. As long as a 

 stem develops its foliage under water, the leaves of these plants 

 at once assume the delicate elongated form so characteristic of 



eudogens; whereas in Victoria the submerged leaves (like those 



of Ranunculus heterophyllus) are adopted from a more adult 

 stage of growth. Consequently the first leaf in the Nymphceacea 

 is not flattened out and ribbon-like, but more res ambles a chan- 

 nelled petiole without a lamina. 



From the above considerations one arrives at the conclusion 

 that those terrestrial eudogens which still retain a linear form 

 (e. g. Grasses) or ensiform (Iris) or other similar type of leaf, or 

 rather phyliode, may be regarded as representing the ancestral 

 submerged ribbon -like form; and those endogens with distinct 

 blades of a lanceolate shape (Convallaria, Epipactis, Paris, &c.) 

 or cordate (Maianthemuni) represent the floating types of existing 

 aquatic endogens ; while the sagittate form, so characteristic of 



"A Theory of the Forms of Floating Leaves in certain Plants," Proc. Camb. 

 Phil. Soc. pt. xiii., 1872. 



