272 MANUAL OF BOTANY 



present in all seeds ; it is developed from the fertilised oosphere 

 in the embryo-sac. The embryo being the rudimentary sporo- 

 phyte, it is necessarily the most important part of the seed, and 

 it contains within itself, in an undeveloped state, all the essential 

 parts of which the plant is ultimately composed. Thus we dis- 

 tinguish in it three parts ; namely, a radicle, plumule, and one 

 or more cotyledons. These parts may be readily recognised in 

 many seeds ; thus in the embryo of the Lime {fig. 590), the 

 lower portion, r, is the radicle, from which the root is developed ; 

 the two expanded lobed bodies above, u, c, are the cotyledons ; 

 and between these the plumule is placed. In the Pea, again 

 (fig. 16), the two fleshy lobes, c, c, are the cotyledons, between 

 which there is situated a little axis, t (tigellum), the upper part 

 or budlike portion of which is the plumule, n, and the lower part, 

 r, the radicle. Plants which thus possess two cotyledons in their 

 embryo are called dicotyledonous. But there are plants in which, 

 as already noticed, there is commonly but one cotyledon present 

 (figs. 591 and 593, c), and which are accordingly termed mono- 

 cotyledonoiis. In rare instances, however, the embryo of a plant 

 classed among the Monocotyledons has more than one cotyledon, 

 but then the second cotyledon alternates with the first, instead 

 of being opposite to it, as is invariably the case with the two 

 cotyledons of dicotyledonous plants. The phyllotaxis of the 

 Monocotyledons, as exhibited in the embryo, is thus always 

 alternate, while that of the Dicotyledons is opposite. In the 

 Gymnosperms there are often several cotyledons, so that tbe 

 embryo is described as polycotyledonous. 



(a) The Monocotyledonows Embryo. — The parts of the mono- 

 cotyledonous embryo are in general by no means so apparent 

 as those of the dicotyledonous. The embryo at first sight, 

 externally, usually appears to be a solid undivided body of a 

 cylindrical or somewhat club-shaped form, as in Triglochin (fig. 

 593) ; but if this be more carefully examined, a little slit, /, 

 or chink, will be observed on one side near the base ; and if a 

 vertical section be made parallel to this slit, a small conical 

 projection A^'ill be noticed, which corresponds to the plumule : 

 by making a horizontal section, the cotyledon will be noticed 

 to be folded round the plumule, which it had thus almost 

 entirely removed from view, only leaving a little slit correspond- 

 ing to the union of the margins of the cotyledon ; this slit thus 

 becomes an external indication of the presence of the plumule. 

 In fact, the position of the cotyledon thus rolled round the 

 plumule is similar to that of the sheaths of the leaves in most 



