516 KEV. GEORGE HENSLOW ON A THEORETICAL 



tively, as the surface of tl\e water is approached, the spathulate 

 and hastate forms, Finally, there occurs the sagittate type, but 

 this is now aerial. The sagittate-peltate is attained by Hichardia, 

 but the orbicular form is not reached in these genera. M. Co- 

 stantin figures an individual plant of Sagittaria which bore, after 

 a small number of submerged leaves, some floating oblong-cordate 

 leaves ; but the truly circular form is, however, characteristic of 

 the endogen HydrocJiaris and imitates that of Limnanthemum, 

 Nupliar, Cctltha, &c. among exogens. Conversely, the ribbon- 

 form so characteristic of endogens is imitated b} r those of Lobelia 

 Dortmanni, Limosella aquatic a, Littorella lacustris, Hipp ur is 

 vulgaris, and Callitriche in exogens. 



The Forms of Aquatic Leaves dl t e to Self-adaptation to 

 the Entironment. — That all the forms of leaves, submerged or 

 floating, represent so many adaptations to their environment, is 

 particularly well seen in the formation of the linear-ribbon-like 

 form of endogens and the dissected type of exogens ; for these 

 can be produced at will. Thus M. Costantin has shown that 

 if a leaf of Sagittaria which has begun to form a sagittate blade 

 near the surface be suddenly plunged into deep water, the form 

 is instantly changed ; the blade now develops an elongated point, 

 attempting, in fact, to return to the deep-water ribbon-form. It 

 ultimately assumes an elongated-hastate shape. I have, however, 

 a specimen in which all the three points appear to have been 

 affected ; and although the blade is sagittate, the points have 

 grown out into elongated and narrow prolongations. 



As another adaptation, I have found grasses growing com- 

 pletely submerged, the leaves of which had assumed the ribbon- 

 form, while the anatomy was the same as that of ordinary and 

 normal ribbon-like types. 



Similarly with Ranunculus lieterophyllus, leaves can often be 

 procured, one half being of the floating type, the other half of 

 the leaf being dissected; and if an aerial terrestrial plant with 

 dissected leaves be plunged into water, all the adult leaves at 

 once perish, new ones in adaptation to water soon taking their 

 place ; but all half-developed leaves continue to grow, their new 

 growths, however, conforming to the watery medium. 



That the floating leaves also assumed forms in adaptation to 

 their conditions is presumable from the fact that similar forms 

 are found in widely different plants, as mentioned above ; and 

 such, therefore, cannot be regarded as any indication of affinity : 





