36 



BOTANICAL GAZETTE 



[JANUARY 



The principle of homoeosis 



But whether or not the transference of absciss-layer from leaf to 

 leaflets in this fern verifies in any material degree the idea of a trans- 

 locational origin for leaflet-abjection, it alone, were there no other 

 available demonstration, would suffice to establish the important 

 truth, that a character perfected in the course of evolution under one 



■ 



relation in the plant body may make its appearance suddenly under 

 another relation and in a region of the body to which it is not native. 

 Here is a principle of wide import in morphogenesis, and not without 

 bearing — as has just been shown — upon some of the minor problems 

 of plant evolution; concerning which principle some further obser- 

 vations may be offered. 



The described involution of frond-plan belongs to one among 

 several categories of homoeosis all subsumed under the larger type, 



but of this type it is a rather com- 

 plicated example. A simpler illus- 

 tration will make the nature of the 

 general principle clearer, and at the 

 same time exhibit its distinctness 

 from another aspect of morphogene- 

 sis with which it is frequently con- 

 fused — that of reversion. 



In 1906 there was discovered in 

 the county of Dorset, in England, a 

 solitary plant of Platanthera viridis, 



Fig. s.-Abnormal flower of P/a- th ° fl ° Wer ° f whkh WaS remarkable 

 tanthera viridis, with three spurs. ^ or the possession of three Spurs 

 After Hemst.f.v 



a 



(fig- 5) 



normal 



one under the lip in the 



the lateral sepals. 3 The extraordinary spurs entirely resembled the 

 normal one. The general conformation of the flower was very 

 nearly that of the species, though slight modifications were observable 

 throughout. 



The conditions of the occurrence make it highly probable that 

 we have here the offspring of normal parents, rather than one mem- 

 ber of a long series of gradually deviating forms; for in a botanicaUy 



s Hemsley, W. B„ Journ. Linn. Soc. 38:3. l9 o 7 . 



