t BOTANY. 37 



extremity. The cotyledons, however, are not always of the same size, and 

 the union between the two may be so intimate as to give rise to the pseudo- 

 monocotyledonous embryo. Sometimes there are more than two cotyledons, 

 and plants in which this occurs are sometimes termed polycotyledonous. Co- 

 tyledons are usually entire and sessile. Sometimes, however, they become 

 lohed as in the walnut, petiolate, or miriculate. Like leaves in the buds, coty- 

 ledons may be either applied directly to each other, or else folded in various 

 ways, becoming conduplicate. reclinate, convolute, circulate, &-c. 



The radicle may be either straight or curved, the difference in this 

 respect characterizing certain divisions of plants. Thus, in Cruciferas the 

 division Pleurorhizeje exhibits the cotyledons applied by their faces, with the 

 radicle folded along their edges, so as to be lateral ; the cotyledons are then 

 accumbent. In Notorhizfe the dorsal radicle is folded on the back of the 

 incumbent cotyledons, these being applied to each other by their face. In 

 OrthoplocefE the cotyledons are condupHcate, and the radicle included between 

 their folds. 



With respect to the perisperm, the embryo is internal or intra? ins when 

 inclosed by this on all sides, excepting the radicular extremity ; when lying 

 outside of the perisperm, and only coming in contact with it at certain points, 

 the embryo is external or extrarius. When the embryo follows the direction 

 of the axis of the seed, it is axile or axial, and may be either internal or 

 external. When the embryo is not in the direction of the axis, it is abaxial. 

 When, as in some campylotropous ovules, the embryo is curved and external 

 to the perisperm, it is peripherical. 



Although the radicle is usually turned towards the micropyle, and the 

 cotyledons to the chalaza, yet the former may be directed to one side of the 

 nucleus, and the embryo is then excentric. The position of the embryo in 

 different seeds varies. In an orthotropal seed the embryo is antitropal, the 

 radicle pointing to the apex of the seed ; if the nucleus be inverted or 

 antitropal, the embryo will be erect or orthotropal. In curved seeds the 

 embryo is folded, so that the extremities are approximated, hence called 

 amphitropal. 



When a seed begins to germinate, the embryo first lengthens its radicle, 

 then its caulicle. and afterwards sends the plumule upwards, in the form of 

 a stem and leaves. The radicle extends downwards, either directly from the 

 base of the embryo, or after having previously ruptured the integument of 

 the base. Plants with the first character are said to be exorhizal ; with the 

 second, endorhizal. The former is most common in dicotyledons ; the latter, 

 in monocotyledons. In most plants the cotyledons are gradually jaised to 

 the surface by the growth of the caulicle, after which they become green 

 and act as leaves ; sometimes, however, the cotyledons remain inclosed within 

 the testa. 



/. Reproductive Organs of Flowerless Plants. 



We have already adverted in brief terms to many of the peculiarities of 



37 



