64 PLANT-LIFE ON LAND [CH. 



borne on the ends of lateral branches, and terminated 

 their growth just as those of Seed-plants do nowa- 

 days. The foliage resembled that of the present-day 

 Cycads. The most interesting point in these flowers 

 as bearing on our present discussion is the similarity 

 of their stamens to the fertile leaves of certain Ferns. 

 The view which may be taken of these primitive 

 flowers is that they represent a region of the Fern- 

 like shoot which has retained its function of pro- 

 pagation, while the region below is sterile and is 

 developed to carry on the other pristine function of 

 nutrition. 



Such an example as this indicates the origin of 

 the propagative organs at the centre of the flower. 

 But so far no explanation has been given of the origin 

 of the outer envelopes, the calyx and corolla : nor 

 is there any certain knowledge how they came into 

 being. It may be that they originated in different 

 ways in the various groups of plants. In some cases 

 it seems probable that the calyx sprang from bract- 

 like leaves of the vegetative region already sterile, 

 which became specialised for a protective duty. On 

 the other hand, the corolla in certain cases was 

 probably derived from the outermost stamens which 

 became diverted from their specific function as pro- 

 pagative organs to the duty of attraction of insects 

 by colour, scent, and honey-secretion. By such means 

 a further step of sterilisation appears to have been 



