COTYLEDONS. 



in the mud under 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 u and 144 15 ), while in species 

 of Yucca and Tradescantia it grows downward in an arch (see fig. 141 9 ); and in 

 cycads and palms, growing in soil exposed superficially to drought, it bends round 

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

 layers of earth which are always somewhat moist (see figs. 144 7 ' 9 ' 10 ). In the 

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

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

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

 the bud were connected by a long thread with the absorbent 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 in the 

 cycads Zamia, Ceratozamia, Encephalartos. 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 covering 

 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 behind in the seed forms a hollow cone which becomes 

 dilated like a vesicle where absorption of the reserve materials occurs (figs. 144 9 

 and 144 10 ). 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 8 ). Gardeners employ 

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

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

 dibbles in observing how the tubular, rolled, stalk-like cotyledon-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 planted in a suitable place actually in the most favourable 

 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 8 kilograms, are conducted by this sheath to the embryo planted below. 

 Numerous species of Onion (Allium), and of Reed-mace (Typha) exhibit 

 our fourth form of cotyledon. The extrusion of the embryo 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 17 ). 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 hypocotyl and the bud, comes to lie below the 



