ORGANS OF REPRODUCTION. 



347 



of the cotyledon ; and which slit thus became an external indi- 

 cation 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. 687), 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. 



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 they are 

 always opposite to each other if there are but tAvo cotyledons, or 

 whorled (fig. 750), when they are more numerous. 



The inferior extremity of the radicle is usually rounded 

 (fig. 745, r); and it is through this point that the roots burst 

 in germination (fig. 742, r). The radicle is usually much 

 shorter than the cotyledon, and generally thicker and denser in 

 its nature ; but in some embryos, on the contrary, it is as long 

 or even longer, in which case, the embryo is termed macropodous. 



b. The Dicotyledonotis Embryo. — These embryos vary very 

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

 Bean and Almond (fig. 746), where the embryo consists of two 

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



Fig. 746. Fig. 747. 



Fig. 748. Fig. 749. 



Fig. 746. The embryo of the Almond 



(.Amijgddlns communis') from which 

 one of tlie cotyledons has been re- 

 moved, c. The cotyledon which has 

 been left. r. Radicle, g. Gemmule. 

 t. Tigelle or caulicule. c'. Scar left 



by tlie removal of the other cotyledon Fig. 747. Vertical section of the 



embryo of a species of i7(rcErt. c'. Large cotyledon, c. Small cotyledon, g 



Gemmule. r. Radicle Fig. 748. Vertical section of the embryo of Ca- 



rapa Guianensis, showing the almost complete union of the cotyledons, the 

 line, c. only dividing them. r. Radicle, j?. Gemmule -fV^r. 749. The em- 

 bryo of Fekea butyrosa. t. Large tigelle. c. Rmiimentary cotyledons. 



the upper part of which is the plumule, g, and the lower the 

 radicle, r, the point of union between the radicle and cotyledons 

 being called the caulicule or tigelle, which upon germination 

 appears as a Uttle stalk supporting the cotyledons. 



