518 REV. GEORGE HENSLOW ON A THEORETICAL 



the Aroidece, has primarily arisen, like the same forms in the 

 Nympliaacece and Sar/ittaria, from an aquatic habit. 



Origin of the Aloeidal type of Leaf. — A particular ex- 

 ample of leaf may he here mentioned, namely that of Stratiotes 

 aloides. This plant ha3 leaves of the ribbon-form provided with 

 what I would call " dog-tooth " cells at intervals along the mar- 

 gins. They have nothing in common with the teeth of an aerial 

 serrated leaf, as that of a rose, or a dentate aerial leaf. Such 

 projecting cells are not uncommon on submerged leaves, and may 

 even take the form of subulate bristles, as occurs on the tips of 

 the segments of Ranunculus heterophyllus and in the " traps " of 

 Utricularia. 



Comparing the foliage of Stratiotes with terrestrial forms, we 

 seem to see it repeated in Pandanus, the leaf of which has parallel 

 venation with rectangular cross-bars. It is also provided with 

 very similar teeth. "We may. perhaps, advance a step further 

 and recognize the same submerged type of leaf as the origin of 

 that of Aloes, which have supplied the specific name to Stratiotes , 

 although these plants have now acquired an exactly opposite 

 habit of life to an aquatic. 



The " Junceous " type. — There is yet another type of leaf 

 which may be briefly mentioned here which is more or less cha- 

 racteristic of certain orders of endogens, but of certain aquatic 

 plants only of exogenous orders. This is the well-known jun- 

 ceous type, in which the leaf is not infrequently fistular and sep- 

 tate. This occurs in species of Juncus which furnish the name, and 

 may be also seen in (Enanthe Jistulosa and Grant zia lineata and 

 many other plants. As it is associated with an aquatic habit, the 

 inference is justifiable from all analogy that it is the watery en- 

 vironment which has brought it about. 



Ve^atio^. — It will now be desirable to give a brief series of 

 forms to show, on the one hand, how terrestrial endogens have 

 arisen from aquatic endogens ; and, on the other, how these latter 

 arose from aquatic exogens. A study of the venation will illus- 

 trate this process remarkably well. 



In comparing the venation of the leaves of terrestrial endogens 

 with that of aquatic endogens, and again with that of some 

 forms of aquatic exogens, we at once see very close resemblances ; 

 bo that a series can be readily made from extreme simplicity, 

 as of Hippurisj Zostera, or Potamogeton gramineum, to Tamus 



