7^ PROCIOKUINCiS OF TIIIO 



.Moiiocotylt'tloiious leaves whicli ])ossess a distinct stalk and 

 blade, is merely an expansion oF the Jipical region of the petiole, 

 and that thus the " aerial reticulated leat'-blades oF jMonocotyledons 

 are not identical, but only imitative of the tibro-vascnlar system of 

 an ordinary dicotyledonous leaf.." This interpretation, as 1 

 have shown in the jjuper cited*, throws Jight upon a number of 

 peculiar features in the leaf morphology of Monocotyledons, it 

 suggests a possible cause, for instance, for a certain gentral 

 similarity of form which pervades the leaf-bhides of this Chiss. 

 Their curiously cumbrous methods of approximating to the eom- 

 ])0und lamina of the Dicotyledon, and their liability in many 

 cases to be torn to shreds i)y the wind, also harmonise with the 

 view that they are not true lamina? but mere expansions of the 

 petiole or leaf-sheath apex, in which the shnpe and structure are 

 determined by the mode and degi'ee of separation of the originally 

 parallel veins. In the paper to which reference has been mude, 1 

 have also brought forward certain anatomical evidence in support 

 of Henslow's view tiiat the blade of the Monocotyledonous leaf is 

 what I pro]iose to call a ' pseudo-lamina.' If 1 ids interpretation be 

 accepted, it affords confirmation of the vnlidity of tiie ' Law of 

 Loss,' since we have here an instance of a discarded organ being 

 replaced by a modification of another, instead of being reacquired. 



Whatever view be taken of the immediate origin of Monoco- 

 tyledons, they are very generally regarded as descended i'lom a 

 stock which at some period possessed normal iiecondary thickening; 

 if rhis hy])()thesis be accepted, their stem structure m:iy be claimed 

 as exemplifying the ' Law of J^oss.' Interfascicular cambium is 

 typically absent in this Class, and those members which have 

 adopted the tree habit — with the correlated necessity for increased 

 girth — have in no case reverted to a typical cambium formed by the 

 union of intrafascicular meristems ; tliey seem, on the contrary 

 to have <:;roped, as it were experimentally, after new methods of 

 secondary thickening. 



The ])ollination methods of submerged Angiosperms may also 

 possibly be re<^arded as illustrating the 'Law of Loss.' The 

 ciliation of the male gamete in the great group of the Pterido- 

 ])hyta — from which it is supjxjsed that the Flowering Plants are 

 ultimately derived — is associated essentially with aquatic fertili- 

 sation ; with the adoption of terrestrial life this feature was lost, 

 and is now unknown either in the higher Gj'mnosperms or the 

 Angiosperms. It, might well have been expected that when certain 

 A ngiosperM)s adopted water-life so completely as evevi to revert to- 

 the remotely ancestral habit of submerged fertilisation, they would 

 also simultaneously revert to ciliated sperms associated with a broad 

 stylar canal and open inicropyle. Such a trumpet-shaped stigma as 

 that possessed hy ZannichcUia seems, indeed, exactly adapted for the 

 entry of swimming sperms. But no such ciliated Angiospermic 

 gametes have come into t-xistence ; those Flowering Plants which 

 are pollinated beneath the water, go through all the processes of 



* Arber.A. (1918). 



