GOO COTYLEDONS. 



to the gnnving parts of the embryo, lu order to do this, it is necessary that those 

 cells of the cotyledon which adjoin the special reserve- tissue should have the i^ower 

 of absorbing organic compounds from it, and of leading them away. The cotyle- 

 dons in this respect resemble the suckers of parasites, and, like these, are provided 

 with absorbent cells. In many species, ejj. in the Com-cockle (see fig. 141") 

 the}' remain short, form a continuous cell-layer which borders on the special 

 reserve-tissue, and remind one of the absorbent cells of the Bird's Nest (fig. 16-); 

 in others, as, for example, in Tradescantia (see fig. 141^^), they appear as 

 papilljB, slightly separated from one another at the sides, and resembling the 

 absorbent cells of a Gentian root (c/. fig. 16^); again, in other instances, as, 

 for example, in the Wheat (fig. 141 "), they increase, at the time of absorp- 

 tion, to ten or twelve times their previous length, and then their side-walls 

 separate from one another so that they are comparable to the absorbent cells 

 of CusciUa (fig. 35 "). If the embryo is entirely embedded in the special 

 reserve-tissue, it may happen that all its superficial cells in contact with the 

 food-containing tissue, and not only those on the exterior of the cotyledons, 

 act as absorbent cells. If, on the other hand, the embryo only adjoins the 

 reserve-tissue on one side, the absorbent cells also are only developed on this 

 side. The embryo of the Corn-cockle, which is bent like a horse-shoe around the 

 si^ecial reserve -tissue (fig. 141^), exhibits, for example, absorbent cells only on the 

 lower side of one of its two cotyledons, which is directed towards the middle 

 of the seed. Frequently only a very small jiart of the cotyledon possesses 

 absorbent cells adjoining the reserve-tissue, as, for example, in the Onion, where 

 only the end of the cotyledon bears absorbent cells (figs. 141 1». is. i9) ; or in 

 Tradescantia, where the end of the cotyledon presents a knob-like absoi-bent 

 tubercle (fig. 141 "). It is worthy of notice that in many instances where the 

 reserve-tissue is ample and the embryo very small, the extent of the absorbent 

 surface of the cotyledon becomes enlarged during germination. As the reserve 

 materials are absorbed, and the exhausted reserve-tissue shrinks, the absorbing 

 portion of the cotyledon advances. The knob-like termination of the cotyledon 

 of Tradescantia, originally of small dimensions, becomes larger in proportion as 

 the reserve-tissue diminishes. The absorbent, hollow, conical or inflated end of 

 the cotyledons of many palms, e.g. of the Date and Cocoa-nut Palms, increases 

 in proportion as the reserve-tissue diminishes, presses forwards just as far as the 

 tissue to be absorbed shrinks back, and occupies the space vacated by it (figs. 

 144'' and 144'"). A similar relation is to be seen in rushes and sedges. In 

 the embryo of Coffee and Ivy seeds, the cotyledons are at first very small, 

 but grow further and further into the reserve -tissue during the process of 

 germiuation, till they gradually fill up the whole space in the seed. The 

 cotyledons of umbelliferous plants also behave in a very characteristic manner. 

 The small embryo lies in the seed at the base of the reserve-tissue, and its 

 minute cotyledons project into a space occupied by empty cells, which are 

 however, surrounded by the well-filled cells of the reserve-tissue. Xow when 



