304 THE STRUCTURE OF FLOWERS. 



the latter, it is due to reversion by arrest, and is likewise 

 accompanied by a simpler origin of the nncellus and embryo- 

 sac, as Warming has shown. The suggestion I would offer 

 to account for this anomaly is that such ari'est is due to 

 compensation. The Gamopetalce, as a whole, are the most 

 advanced of all flowers through adaptations to insect agency ; 

 and as this invariably brings about an exalted condition of 

 the corolla and stamens, the consequence is that the pistil 

 has to suffer ; the first visible result being protandry, accom- 

 panied by a temporal arrest in the development of the pistil. 



If this tendency to arrest be carried to the ovule, it may 

 be affected too, and the result is that one, the last-formed 

 coat, may be arrested altogether. Orchids, as shown above, 

 illustrate this principle remarkably well, as their ovules, 

 though possessing two coats, are as degenerate as in many 

 parasitic plants (see above, pp. 172 and 281). 



Tracing the origin of an ovule, then, from its birth, it 

 first appears as a papilla on the placenta of the carpel. A 

 branch from the marginal fibro-vascular cord of the carpel 

 enters it from below, and reaches afc least as far as the 

 cbalaza, or base of the nucellus. It may go no further, as in 

 rudimentary ovules of Orchis ; or be arrested in the form of 

 cambium in the degraded state seen in the parasitic Thesium. 

 On the other hand, as the ovule becomes a seed and the coats 

 go to form the seed-skin, fibro-vascular branchings may occur 

 all through the l^itter, being developed from the original cord. 

 Such may be well seen in Mustard, Acorns, Beans, and the 

 Coco-nut. 



Although the coats of the ovule were originally formed 

 by tangential division of the cells of the epidermis of the 

 nucellus, when united to form a seed-skin, this has become 

 thickened by a cellular growth between them, through which 

 the cords then ramify. 



