LIN'XK.VN SOCIETY OF LONDON. "J I 



to indicate the (jeneral rale, that a stnictiwe or onjaii o)ice Io!it in the 

 i'ourse of />h!/Io(/enif can never he regained; if the on/anisin sidj- 

 ^erpienth/ has occasion to replace it, it cannot he reproduced, hut must 

 he constructed afresh in some difrerent mode 



From its very nature, ,8uch a law is not susopptible of formal 

 proof. Its probable validit}' may, however, be inferred if it can be 

 shown that in a number of cases it opens the door to a ratioiuil 

 iaterpretation of features wliich are not easily explicable on other 

 ffroinids. Si) I propose now to consider certain special instances 

 ill which the ' Law of Loss' seems to throw light on the meaning 

 of structural features which would otherwise be obscure, 

 beginning with a case among submerged plants which happened 

 to be one of the first to arrest my attention in this connexion. 

 Cei-atophi/Uiim deinersnm and certain species of Utricutaria are 

 entirely ri)otless at all stages of their life-histor}", even the primary 

 root of the seedling being either altogether absent or remaining 

 quite rudimentary. We cannot avoid the belief that these plants 

 -are descended from ancestors of the normal Angiospermic type, 

 characterised by possessing a root system ; bub tliey have them- 

 selves entirely lost the ancestral capacity for producing roots. 

 Nevertheless, in both these unrelated genera, the need for an 

 absorbing organ seems to have re-asserted itself, and to have 

 been uiet, not by the roestablishment of root formation, but b\' 

 the development of special subterrauean shoots which — though 

 not of the morphological nature of roots — perform a root-like 

 function *. This behaviour in tlie case of Ceratophijllitm and 

 Utricidaria may be interpreted to mean that a plant which has 

 ■entirely given up root-formation and afterwards again experiences 

 the need of roots, cannot reacquire them, hut can only press 

 some existing organ into the service, niodifving it as best it may. 

 It is possible that the root-like water-leaves of Sidvinia indicate a 

 similar history. 



As another instance of the working of the 'Law of Loss,' we 

 may take the phylogenetic history of the leaves of the Alismaceae 

 or Pontederiacete — or, indeed, the leaves of anj^ other Moiioco- 

 tvledons which possess ' laminae.' But whether or no this 

 illustration be accepted, depends upon the standpoint ado|)ted 

 regarding the general morphology of the leaves of Monocotyletlons. 

 In a recent papert, I have dealt at some length with this (Question 

 and have concluded — following de CandolleJ — that the original 

 type of Monocotyledon. )us leaf consists either of petiole and leaf- 

 base, or of leaf-base alone, and entirely lacks a lamina. From 

 this point of vievv, the ' lamina' of a Monocotyledonous leaf, in the 

 cases where it occurs, cannot be interpreted as homologous with 

 the lamina of a Dicotyledon, since, on the Law of Loss, the blade 

 oace discarded can never be regained. Professor Jlenslow§ has 

 ])ropou!ided the theory that the so-called ' lamina ' of those 



* Gliick, H. (190(1). t Arber, A. (11)18). 



: Caiululle, A. P. de(lS27). § lleusiow, G. (1911). 



