

FUNDAMENTAL OBGANS. 123 



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

 and acotyledons 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* Becent experiments of Fremy seem to show that the 

 chemical composition of the cuticle is analogous 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 cell does not retain its form arid 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 caulide. 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- 

 imposed cells, break the transverse walls which separated them, 

 a,nd become vessels. This change takes place in definite posi- 

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

 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 

 being established between this ring and the disk by radiating 6g] ^ elon 



bands of cells (EM), extending from the centre to the cir- Homontauiice of the stem 

 cu inference, 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 in 

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



