144 ON SEEDLINGS 



the embryo, which develops very rapidly, and they are ulti- 

 mately not merely coiled, but frequently interlaced. The whole 

 embryo is extremely delicate or fragile in its earliest stages, and 

 the extreme slenderness of the cotyledons compared with the 

 stout radicle facilitates their being coiled in order to accom- 

 modate themselves closely to the interior of the small round seed 

 without waste of space. After germination and when fully grown 

 the cotyledons of Lepidium sativum. are so deeply tripartite, 

 each segment being narrowed to the base, that they appear 

 as if digitately compound. The probable origin of their 

 present form has already been suggested, ante p. 52. 



The dispersal of the seeds is in many cases aided by the 

 presence of wings ; in some by the agency of animals ; in 

 others they are thrown to a greater or less distance by the 

 plant itself. 



In the latter cases the walls, when the pod is ripe, are in a 

 state of tension. The seeds are loosely attached to the central 

 piece by short stalks. When the proper moment has arrived, 

 the outer walls are kept in place by a delicate membrane, 

 only just strong enough to resist the tension. The least touch, 

 for instance, a puff of wind blowing the plant against a neigh- 

 bour, detaches the outer wall, which suddenly rolls itself up, 

 generally with such force as to fly from the plant, thus jerking 

 the seeds to a distance of several feet. 



Matthiola incana, E. Br. (fig. 156). 



Primary root long, tapering downwards, with a few short lateral 

 rootlets. 



Hypocotyl short, erect, terete, tapering insensibly into the 

 root, glabrous, pale green, covered with small crystalline elevations, 

 8-12 mm. long. 



Cotyledons broadly oblong-oval, obtuse and rounded at the end, 

 entire, petiolate, glabrous, light opaque green, with a scarcely visible 

 midrib, but no other discernible venation in the fresh state ; lamina 

 9-11 mm. long, 7-9 mm. wide ; petiole grooved above, convex on the 

 back, glabrous with a line of small crystalline elevations on the 

 under side, dilated and slightly connate at the base, 7-9 mm. long. 



Stem erect, terete, herbaceous, biennial, densely covered with 

 much branched and radiating stellate hairs ; 1st internode 1- 

 1-5 mm. long ; 2nd undeveloped; 3rd l'S-2'5 mm. long. 



