606 COTYLEDONS. 



in the imul muler water, the extruded portion of the cotyledon surrounding the 

 embryonic .stem, with its bud and first shoot-leaves, becomes bent upwards after it 

 has issued from the interior of the seed (see figs. 144 ^* and 144 ^*), while in species 

 of Yiocca and Tradescantia it grows downward in an arch (see fig. 141^); and in 

 cycads and palms, growing in soil exposed supei-ficially to drought, it bends round 

 immediately after its exit from the seed, and penetrates vertically into the deeper 

 layers of earth which ai'e always .somewhat moist (.see fig.s. 144 ^•^•'''). In the 

 Areca-palm and the slender CJiamcedorea the sheath-like extruded portion of the 

 cotyledon is very short, while in the Commelynacese it is much elongated, so much, 

 indeed, that it looks as if the sheath-like portion surrounding the hypocotyl and 

 the bud wei'e connected by a long thread with the absorljeut portion which 

 remains behind in the seed. This central portion of the cotyledon is also 

 much elongated in the Date palm and in the Cocoa-nut palm, as well as iu the 

 cycads Zamia, Ceratosamia, Encephcdartos. The figs. 7, 8, 9, 10 of the illustra- 

 tion opposite show all the stages of development in the Date seedling. As long as 

 the cotyledon has not pushed out from the interior of the seed, it forms a mantle- 

 like envelope for the bud of the hypocotyl, and is continued into a sac-like covei-ing 

 for the radicle. At germination the cotyledon increases much in length; the 

 free end is sheath-like, the middle portion forms a stalk-like, rolled-up structure, 

 and the part remaining liehiud in the seed forms a hollow cone which becomes 

 dilated like a vesicle where absoi-ption of the reserve materials occurs (figs. 144® 

 and 144^°). In a still later stage the radicle develops into a root, and breaks 

 through its sac-like covering, while the scale-leaves of the epicotyl stretch, and 

 push their way out of the cotyledonary sheath (fig. 144®). Gardeners employ 

 what they call a " dibble ", a tool by the help of which the seeds and seedlings ai'e 

 planted in a suitable depth of earth. One is involuntarily reminded of these 

 dibbles in observing how the tubular, rolled, stalk-like cotj'ledon-sheath — which 

 grows out of the seed — not only pushes the embryo out of the interior, but presses 

 it deeper and deeper into a layer of earth which by its depth is protected from 

 drying up; there it is jjlanted in a suitable place — -actually in the most fav^ourable 

 position. In many palms the cotyledonary sheath is half a metre long, and many 

 months pass before all the reserve-materials of the gigantic seed, often weighing 

 as much as S kilograms, are conducted by this sheath to the embryo planted below. 

 Numerous species of Onion {Alliimi), and of Reed-mace {Typlia) exhibit 

 our fourth form of cotyledon. The extrusion of the embrj^o by the cotyledon 

 is conducted in the same way as in the type just described, but there is this 

 essential difference, that here the cotyledon, after it has absorbed the reserve- 

 materials of the seed by its apex, entirely vacates the cavity of the seed-coat, 

 becomes green, and then acts like a foliage -leaf. In the seed of the Garlic 

 {Allium sativum) the embryo is embedded in the centre of the reserve {cf. fig. 

 141 1^). As soon as germination begins, the cotyledon pushes its way out of the 

 seed-coat, and grows first upwards, then bending round at an angle, so that 

 the extruded end surrounding the hj'jiocotyl and the bud, comes to lie below the 



