156 MORPHOLOGY, OB COMPARATIVE ANATOMY. 



tissue loaded with fixed oil (as in the Poppy and Cocoa-nut); muci- 

 laginous or fleshy when it is tougher and swells up readily when 

 wetted (as in the Mallow) ; horny when hard and more or less 

 elastic (as in Coffee, Galium, Iris, &c.). 



The perisperm is usually a uniform mass ; but in Nympheea, Piperacese 

 (fig. 318), Canna, and some other plants the embryo is contained in an 

 inner central compartment or sac (sometimes called the amniotic sac) 

 so that the perisperm is here double. The enclosed portion is some- 

 times called the endosperm ; the development of this will be describee 

 in the Physiological part of this work. 



The uniformity of the perisperm is also destroyed in some seeds by a 

 peculiar tabulated condition of the outer portion, the sinuosities being 

 rilled up and enclosed in an inseparable layer of different-coloured tissue 

 giving a marbled appearance ; this, which 'is seen in the Nutmeg, is calle( 

 a ruminated perisperm or albumen. In the Cocoa-nut the perisperm 

 hollow when mature, containing the so-called milk (fig. 319). 



The Embryo. The embryo, or rudimentary plant contained in 

 the seed, ordinarily possesses, when the seed is mature, all tht 

 essential organs of vegetation, namely root, stem, and leaves, al- 

 though in a few cases the leaves are undistinguishable ; while in 

 others the embryo is a mere cellular nodule in the ripe seed, as in 

 Orchidacego and Orobanchaceae. The embryo is the result of the 

 fertilization of the germinal vesicle or oospJiere contained in the 

 embryo-sac (p. 139). 



Parts of the Embryo. The end of the embryo usually pointing 

 to the micropyle is the radicle (figs. 320-323, a) or rudimentary 

 root, continuous with the lower end of the axis which termiuates 

 at the other end in the plumule (figs. 321-323, c) or rudimentary 

 terminal bud. The axis itself is sometimes very short, being a mere 

 " collar" between the base of the seed-leaves and the radicle ; but, in 

 some cases, it is developed into a well-marked hypocotyledonary axis 

 or tigellum, distinguishable from the radicle by its cylindrical form 

 (or, if conical, the point of the cone is upwards). The rudimentary 

 leaves, called cotyledons (figs. 321-323, 6", &', //), differ in number in 

 the two great classes of ADgiospermous Flowering plants, since in 

 the Dicotyledons there are two placed face to face at the upper end 

 of the axis, with the plumule between them (fig. 321); and in 

 Monocotyledons only one exists (or the rudiment of another on a 

 different level), and this is more or less completely rolled round 

 the plumule, like the sheath of the leaf in Grasses (fig. 323). 



The embryos of the Gymnosperms are either dicotyledonous, as in 

 Cycas, Taxus, Juniperus, &c., or really or apparently polycotyledonous, as in 

 Pinus (fig. 324), where it is said that the seeming whorl is formed of two 

 deeply divided cotyledons. 



Direction of the Embryo. The embryo, whether covered only by 



