SEED AND EOOT 



169 



and to elaborate starch, etc., for themselves, 

 the plant is dependent upon the supply 

 contained in its two seed-lobes or cotyledons, 

 which, unlike the single cotyledon of a 

 Barley grain, form the chief substance of 

 the seed. 



The young plants of Mustard (Brassica 

 alba). Cress (Lepidium), Poppy (Papaver), 

 etc., which are not thus liberally endowed, 

 are thrown upon their own resources at a 

 very early age, and have to work for their 

 living almost directly they have broken 

 from their shells. In such cases the cotyle- 

 dons rise above the ground very soon after 

 germination has commenced, and at once 

 perform the functions of true leaves developing 

 chlorophyll and taking in carbon dioxide in a 

 business-like and energetic manner. In this 

 way the plants are kept alive and vigorous till 

 ordinary leaves are produced. 



By soaking a Bean in warm water for a 

 short time, the thick double skin or testa, with 

 which it is surrounded, may be easily removed, 

 and the two large fleshy lobes, which are the 

 cotyledons of the embryo, may then be separated 

 without difficulty, and the plumule and radicle 

 laid bare (fig. 212). Before stripping the 

 seed, the small black scar or hilum should be 

 noticed ; as well as a minute aperture at one 

 end of it, the micropyle, from which a small 

 quantity of water may be expressed if the moist 

 seed be squeezed between the finger and thumb. 

 When the testa has been removed and the 

 cotyledons thrown open, the root of the germ- 

 plant will be seen to be directed towards this 

 aperture. 



On stripping a seed of Maize (Zea mays), a little 

 examination will show how small a portion of the 

 seed the single cotyledon occupies. Indeed, when 

 the whole of the embryo plant, consisting of 

 plumules, radicle, and cotyledon, has been picked 

 out of the white floury matter in which it is 

 embedded, it will be found that the bulk of the 

 seed remains (figs. 215-217). 



FIG. 207. SPORANGIA OF FERNS. 



(a) Maidenhair Fern (Adiantum capillus-veneris), 

 with spores escaping, (fe) Royal Fern (flsmunda 

 regalis). (c) Bristle Fern (Trichomanes radicans). 



FIG. 208. ROUND - LEAVED 

 WOODSIA ( Woodsia hyperborea). 



Tart of a frond with five clusters of 



sori. Below, a single sorus, consisting 



of a cluster of sporangia. 



FIG 209. PROTHALLUS OF 

 A SPECIES OF MAIDENHAIR 

 FERN SEEN FROM BELOW. 



