136 ON SEEDLINGS 



seeds in this group are oblong, oval, ovoid, or variously angled ; 

 and there is a general absence of the flattened or winged 

 character typical of the first group. In many instances there 

 is a distinct ridge along one side of the seed, generally, 

 if not always, on the dorsal aspect, and in this the radicle is 

 accommodated, the ridge itself finding room in a correspond- 

 ing concave or grooved line along the middle of the two valves 

 of the siliqua. Should the seeds be comparatively large, they 

 are arranged in one line along the middle of the two cavities 

 of the ovary. The various species of Erysimum and Sisym- 

 brium show this well. The carpels of Lepidium are laterally 

 flattened and keeled on the back; each cavity contains one, 

 rarely two seeds ; and these are suspended from the apex of the 

 placenta, and trigonous with the thinnest edge lying in the 

 cavity of the keel as in L. sativum and L. graminifolium 

 (fig. 171) ; in other cases they are laterally flattened but not tri- 

 gonous. The seeds of ^Ethionema heterocarpum are covered 

 with mucilaginous tubercles, and the testa of Lepidium is 

 also mucilaginous when moist, which may assist in retaining 

 it in the soil, while the embryo escapes during germina- 

 tion. 



Several curious modifications occur in this group. The 

 oblong seeds of Isatis tinctoria (fig. 174) are solitary in each 

 fruit, which is indehiscent, and furnished with a curious corky 

 wing along the back of each carpel. The cotyledons are concave 

 longitudinally and slightly embrace the stout radicle. Hesperis 

 matronalis (fig. 162) is unsymmetrical, one of the cotyledons 

 being considerably smaller than the other. The seeds of Sisym- 

 brmm officinale are variously shaped and angled, while the 

 radicle m most cases lies obliquely across the back of the coty- 

 ledons. The same may be said of Ochthodium jegyptiacurn 

 (fig. 175), which has a woody, curiously tuberculated, inde- 

 hiscent fruit. Tetrapoma, a Siberian genus, containing only 

 one species, has a one- or four-celled ovary, according as the 

 partitions are developed to the centre or not, while there are 

 four split or double placentas with the seeds arranged in ei^ht 

 rows. 



Conduplicate cotyledons are typically characteristic of all 

 the eleven genera classed in the tribe Brassiceie ; but they also 



