ORIGIN OF EXDOGEXS FROM EXOGENS. 510 



communis and Arum triphyllum, which have adopted, e. g., the 

 same process of inarching of the veins * near the margin as is 

 so characteristic of exogens, together with a perfect system of 

 reticulations between the stronger ribs. 



Some such series may be formed of the following:— Commencing 

 with submerged leaves, as Hippuris, Zostera marina, and Pota- 

 mogeton gramineum, these have from one to three or more delicate 

 ribs about equal in strength, running parallel to one another from 

 end to end with no cross-bars f . The aerial leaves of Sparganium 

 ramosum are in this condition, while a terrestrial form with greatly 

 strengthened ribs is Narthecium ossifragum. In Potamogeton 

 crispum the leaf is reduced in length and broader ; in Sparganium 

 natansihe leaf is long and narrow; but in both the lateral ribs 

 are now joined by well defined cross-bars, at an angle of about 

 45° in the former and at right angles in the latter. The onter 

 series in the former are connected with a delicate marginal rib. 

 A much broader leaf, framed on the same plan, is that of Ouvi- 

 randra fenestralis, the ribs being equal in strength and united by 

 cross-bars at right angles to them. An exactly similar venation 

 appears to have characterized species of Naiadita occurring in 

 the Liassic strata near Bristol ; while Pandanus, of living trees, 

 and Goodyera repens, of herbs, have a similar venation. 



Passing on to floating leaves, Potamogeton nutans furnishes the 

 next type of venation. It has pointed-elliptical blades with curvi- 

 linear ribs; but instead of having cross-bars, the interstices are 

 now filled in with reticulations. The leaves of Lilium auratum 

 represent this type among land-plants of endogens. We may, 

 perhaps, see an important difference at this point, namely, that 

 the rule may, perhaps, be found that reticulations do not appear 

 unless the leaf is exposed to air and light. On the other hand, 

 the quadrangular spaces between the ribs and cross-bars are the 

 result of being submerged. This appears to be proved by the 

 leaves of certain exogens which are now aquatic in habit, in 

 which the reverse process may be seen to be attempted. Thus, of 



For convenience of distinction I call the longitudinal cords " ribs " and 

 the lateral branchings " veins," forming "cross-bars" and " reticulations." 



t Zostera furnishes a peculiar form of cross-bar ; for it does not merely unite 

 two adjacent ribs, but runs between the two surfaces of the leaf, horizontally or 

 obliquely, across several ribs, always terminating abruptly in the side of a rib. 

 It does not consist of" liber-fibres," of which the ribs are solely coniposed. It 

 is difficult to make out from dried specimens the nature of these M rods/' Tiny 

 fracture transversely easily. They require further investigation. 



