492 ON SEEDLINGS 



at the base with a second pair of strong nerves given off from 

 the middle, and all the four laterals are incurved at their 

 apices. The first two leaves are opposite, trinerved, ovate 

 and serrate-dentate. The third and fourth are rotund-ovate ; 

 all are covered with coarse stinging hairs projecting at right 

 angles to the surface. The cotyledons of Dorstenia Contra- 

 jerva (fig. 647) are rotund, emarginate, rarely oval, subtruncate 

 at the base and trinerved. The primary leaves are cordate, 

 trinerved, distantly dentate, and serrate. They show a ten- 

 dency to become trifid ; and the leaves of the adult are three- 

 to five-lobed. A slight divergence from this type is shown by 

 Ficus bengalensis (fig. 648) which has roundly ovate, emar- 

 ginate and trinerved cotyledons close to the ground owing 

 to the shortness of the hypocotyl. The first two leaves 

 are opposite, ovate and obscurely crenate, while the third and 

 fourth are alternate, and ovate with a cordate base. 



A third type, represented by Maclura aurantiaca, has 

 oblong-oval, entire and somewhat penninerved cotyledons. 

 The two primary leaves are opposite, lanceolate, acuminate, 

 obtuse and penninerved. 



A rather distinct type occurs in Celtis occidentalis (fig. 646) 

 which has oblong, deeply emarginate, trinerved cotyledons, 

 cuneate at the base and tapering into a short petiole. The 

 cause of the emargination has been described above as 

 due to folding. In Urtica dioica (fig. 650) it was due to a 

 thickening at the chalaza inside the seed, and is doubtless 

 augmented by unequal growth subsequent to germination. 

 Two of the primary leaves are opposite, ovate, trinerved, 

 serrate and symmetrical at the base. The four succeeding 

 ones are much larger and wider but otherwise similar. The 

 ultimate leaves are unequal at the base, somewhat resembling 

 those of the Elm. 



The cotyledons of Cannabis sativa are unequal, obovate- 

 spathulate, sessile and trinerved. The longer cotyledon varies 

 from 1-4-1-6 cm. in length and 5-6 mm. in width, while the 

 shorter one is 1-1-3 cm. long and 5-5-6 mm. wide. Their 

 shape and inequality are due to their being incumbent in the 

 seed. The larger one occupies the periphery of the seed 

 and is curved, while the smaller lying between the long one 



