608 



COTYLEDONS. 



brought about by the knee of the upwardly-growing cotyledon acting like a 

 wedge, and thus making a path upwards through the ground. This penetration 

 of the ground is materially assisted by the presence of cells on the convex 

 side of the knee which, unlike the other superficial cells of the cotyledon, are 

 somewhat curved outwardly, and highly turgescent a contrivance which will 

 be described more in detail later on. When finally the free end of the cotyledon 

 has been drawn out of the ground, the knee-shaped bend is obliterated as the 

 green cotyledon straightens out. 



The germination of the Reed-mace (Typha) is quite peculiar. The small 

 fruits which are blown off the spike, fall on to the surface of the water and 

 remain floating for some days. Then the pericarp opens and the seed sinks 

 slowly down into the water. The husk of the seed is pointed at one end, and 

 at the other is closed by an extremely pretty trap-door (cf. fig. 144 11 ). While 

 sinking through the water the pointed end is turned downwards, and the 

 covered end upwards. At the bottom the seed lies in the position indicated 

 and germination commences. The cotyledon grows in length, pushes open the 

 trap-door, and makes its appearance at the mouth of the seed-coat (fig. 144 12 ). 

 It now describes an arch and the end in which are concealed the hypocotyl 

 and the bud reaches the mud. Scarcely has it done so, however, when its 

 epidermal cells elongate and form long tubular structures which penetrate 

 into the slime, and the free end of the cotyledon is thus firmly fixed (fig. 144 13 ). 

 Later on rootlets make their appearance, which, proceeding from the hypocotyl, 

 break through the unresisting cotyledon. Meanwhile the reserve food has been 

 sucked up by the apex of the cotyledon which remained in the seed; this apex 

 is now drawn out of the seed-coat, the cotyledon straightens itself, turns green, 

 and functions as a foliage-leaf. 



In the four cases just described the embryo only possesses one cotyledon, 

 and each seed contains a reserve - tissue beside the embryo. In the fifth case 

 now to be described, however, the embryo is equipped with two cotyledons, 

 and the building materials which are at its disposal for the first period of 

 growth are stored up in the embryo itself, almost entirely indeed in the 

 cotyledons. To this class belong plants with stone-fruits as well as most 

 species with seeds and fruits of nut-like appearance, and many the seeds of which 

 have a softer, leathery covering. As examples may be named the Walnut and 

 Hazel, the Oak, Chestnut and Horse-chestnut, the Almond, Cherry, Apricot, and 

 Peach, the Laurel and Pistachio-nut ; the Nasturtium (Tropoeolum), Broad-bean, the 

 Scammony (Cynanchum), and the Bastard-Balm (Melittis). The two leaves pro- 

 ceeding from the hypocotyl almost completely fill the space inclosed by the seed- 

 coats in all these plants ; and the small embryonic bud as well as the radicle are 

 situated between the two cotyledons, just like a dried plant between the sheets 

 of paper in a herbarium. The cotyledons are thick, swollen, and tense, of fleshy 

 appearance in section, and always comparatively heavy. Many of them are wavy, 

 and they rarely look leaf -like. Occasionally the two cotyledons are fused together 



