^ J.IXXK.VN SOCIEI'V OF LONDON. 73- 



making pulleii grains as if for aerial jioUiiiatioii, willi merely such 

 slight moditirations jis will pern)it them to be carried passively to 

 the stigma by gravity or water currents. It seems that cilia once 

 lost cannot be recovered, even when the circumstances in which 

 they were formerly ot: use again ret-ur, and the plant has, as it 

 were, to patch up some substitute. 



But the application of the ' Lau' ot" Loss ' is by no means 

 confined to water-plants and Monocotyledons. In the case, for 

 instance, of the CompositcB, Dr. Small* has propounded a theory 

 which is very suggestive in connexion with the present line of 

 thought. He supposes that this dominant family has been ulti- 

 mately derived Irom an ancestor among the Lobelioidea; in which 

 the calyx may have been entirely absent. On his view, the pappus 

 of the Compositae is an independent trichome structure. This is 

 exactly what might have been anticipated on the ' Jjaw of Loss.' 

 The ancestral Composite, being unable to reproduce the discarded 

 foliar members, achieved a more or less equivalent result by the use 

 of material belonging to a totally different inorpltological category. 

 Probably the initiation of the ])appus was a valtiable asset in 

 assuring the success of the Compositae It is an illustration of 

 the favoiu'able w ay in which the ' Law of Loss ' may sometimes 

 act in evolution, hy dosiuj/ ivell-wom paths and thus hidirectly 

 eneourariing the openiiKj of frcfh routes. 



Another case may also be considered, although it involves 

 certain premisses which many botanists miglit not concede. A 

 well-developed female |)rothallus characterises the Gymnosperms 

 as well as all the heferosporous Avascular Cryj)togan)s. In the 

 Angiosperms, this prothallial food-tissue has been entirely lost ; 

 this loss was possibly coiuiected with a reduction in size of the 

 ovules correlated with the closing of th'i carpels. It is true that 

 it is held by some that the endosperm of the Angiosperm 

 represents a postponed development of the Gyranosperm pro- 

 thallus, but there is little e\ idence in support of tliis hypothesis. 

 There is much, on the other hand, to recommend Miss Sargant'sf 

 theory that the endosperm corresponds to a second embryo which 

 has been " spoiled "and reduced to a short-lived food-tissue by the 

 intrusion of the second polar nucleus into the fusion of the second 

 male nucleus with the upper polar nucleus — the sister of the egg 

 nucleus. If either Mi>s tSargaiit's theory, or any other theory 

 which does not treat the endosperm of the Angiosperms and 

 Gymnosperms as equivalent, be accepted, we have a case in 

 which a new structure of enlirel}^ different homologies serves the 

 same purpose as a jjieviously existing i)ut now tliscarded organ. A 

 new acquirement, brought ijifo being in this way to replace a pre- 

 existing structure which has been hist, nuiy sometimes be relatively 

 cumbrous and ill-attuned to ])rogressive development {e.y. the 

 anomalous secoiulary thickening of Monocotyledons) or it may, as 

 in the case of the Angiospermic endosperm, prove to be an 



* Small, J. (19 IS) p. SI. 

 t Sargant, E. (KKIO). 



