338 ORGAKOGEAPHT. 



Jection will be noticed, which corresponds to the plumule ; and 

 now, by making a horizontal section, the single cotyledon will 

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

 almost entirely removed from view, only leaving a little slit cor- 

 responding to the union of the margins of the cotyledon ; and 

 which slit thus became an external indication of the presence of 

 the plumule. In fact, the position of the cotyledon thus rolled 

 round the plumule, is analogous to the sheaths of the leaves in 

 most Monocotyledonous Plants, which thus, in a similar manner, 

 enclose the young growing parts of the stem. 



In other monocotyledonous embryos the different parts are 

 more manifest ; thus in many Grasses, as, for instance, the Oat 

 {fig. 689), the cotyledon, c, only partially encloses the plumule, 

 g, and radicle, r ; and thus those parts may be readily observed 

 in a hollow space on its surface {fig. 688). 



We have already stated that a monocotyledonous embryo has 

 occasionally more than one cotyledon, in which case the cotyledons 

 are always alternate, and hence such embryos are readily distin- 

 guished from those of Dicotyledonous Plants, where the coty- 

 ledons are always opposite to each other if there are but two, or 

 whorled {fig. 753, c), when they are more numerous. 



The inferior extremity of the radicle is usually rounded 

 {fig. 748, r); and it is through this point that the roots burst in 

 germination {fig. 745). The radicle is usually much shorter 

 than the cotyledon, and generally thicker and denser in its 

 nature ; but in some embryos, it is as long or even longer, in 

 which case the embryo is termed macropodous. 



b. The Dicotyledonous Embryo. — These embryos vary very 

 much in form: most frequently they are more or less oval, as in 

 the Bean and Almond {fig. 749), where the embryo consists of 

 two nearly equal cotyledons, c, between which is enclosed a small 

 axis, the upper part of which is the 'plumule^ g, and the lower 

 the radicle, r, the point of union, or space between the radicle 

 and cotyledons, being called the caulicule or tigelle {fig. 749, t), 

 which upon germination appears as a little stalk {fig. 15, t), 

 supporting the cotyledons. 



In by far the majority of cases the two cotyledons are nearly 

 of equal size, as in the Pea {fig. 14, c,c), but in some embryos, 

 as in Trapa, some Hir(sas, &e. {fig. 750, c, c), they are very un- 

 equal. Again, while the cotyledons usually form the greater 

 part of the embryo, in other instances, as in Pekea hutyrosa {fig. 

 752, c), they form but a small portion. In the Carapa {fig. 751) 

 again, the two cotyledons become united more or less completely 

 into one body, so that the embryo appears to be monocotyle- 

 donous ; but its nature is readily ascertained by the different 

 position of the plumule in the two cases ; thus in the mono- 

 cotyledonous embryo, the plumule is situated just below the 

 surface {fig. 689, g), but here {fig. 751), the plumule, g, is in 

 the axis of the cotyledons. 



