FUNDAMENTAL OEGANS. 123 



The cuticle is more constantly present than the epidermis ; submerged plants 

 and aeotyledons are clothed in it ; and some botanists have considered that it should 

 be regarded as the true epidermis. Its formation is attributed to the overflow of 

 that intercellular secretive tissue which we have already spoken of as spreading 

 itself upon all the organs, and which deposits a sort of varnish or continuous layer 

 over their outer surface. Eecent experiments, of Fremy seem to show that the 

 chemical composition of the cuticle is analogoiis to that of india-rubber, which makes 

 it a suitable protection for the underlying tissues. Fremy has also discovered that 

 woody fibre is sometimes clothed with a cuticle similar to that which clothes the 

 epidermis. 



FUNDAMENTAL ORGANS. 



We shall now describe the anatomy of the fundamental organs in succession ; 

 i.e. the vegetable axis (stem and root), and its lateral expansions (leaves, sepals, 

 petals, stamens, carpels, ovules). We have described the embryo as a diminutive 

 plant, from which all the parts enumerated above will be developed ; we must there- 

 fore first describe its structure, and then trace its stages of development from its 

 birth till it becomes a plant similar to its parent. 



The embryo invariably commences as a cell with granular contents. In cotyle- 

 donous plants, this ceU does not retain its form and structure ; from spherical it 

 becomes oval ; then at one of the extremities, if the plant is monocotyledonous, a 

 rounded lobe (cotyledon) appears, obliquely and laterally to the axis ; if dicotyle- 

 donous, two lateral lobes {cotyledons) appear, crowning the axis ; the elongated 

 summit of the axis becomes the plumule ; from the opposite end the radicle will be 

 developed, and the body of the cellular mass will form the cauUcle. Following the 

 growth of these fundamental organs, we begin with the stem, which differs remark- 

 ably, according to whether the embryo is mono- or di-cotyledonous. 



Stem of Dicotyledonous Plants.— Take the Melon as a type. In the caulicle, 

 which before germination is entirely cellular, some cells elongate 

 into fibres ; certain of these fibres, together with other super- '>^^^^^L 

 imposed cells, break the transverse walls which separated them, N^^feJ&^^^f 

 and become vessels. This change takes place in definite posi- T^^^^^^^^^" 

 tions, and a horizontal section of the stem (fig. 681) will show ^^^^^^^^m^.r 

 in the centre a disk (m) of large, loose, nearly transparent ^^^^^^^^^ 

 polyhedral or spheroidal cells ; at the circumference, a ring ^^^^^^^^ 

 of dark green, more closely packed cells ; communication -'^s^pa*^ ..^ 



being established between this ring and the disk by radiating esi.Veion. 



bands of cells (rm), extending from the centre to the cir- Horizontal dice oi the stem 

 cumference, and dilating in the same direction ; the whole 

 resembling a wheel, of which the tire is the circle, the axle the central disk, 

 and the spokes the radiating bands. Between the disk and the circle, and 

 separated by the bands, are wedge-shaped plates, which together form a. circular 

 group, and consist of fibro-vascular tissue, and vessels which have been formed ni 

 the middle of the cellular mass, and become united into bundles. The gaping 



