Page:Hardwicke's Science-Gossip - Volume 1.pdf/70

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merely cells combined in several varieties of texture. Now, theorists assume the primary form of cells to be spherical—nature or vital defects in nature (i.e., in growth or development) occasioning deviations, and also causing modifications of the cell where some particular end of plant life is to be accomplished. Thus it is that woody fibre comes to consist of elongated cells thickened by secondary deposits so as to attain great tenacity; and thus also that the ducts and spiral vessels of the vascular tissue, and the peculiar vessels of plants having a milky sap, are cells of special modification, in which the adjoining tubes have often free communication with or run into one another. Indeed, the ultimate results of vegetation, the fruits

Fig. 1.Fig. 2.

of the earth, may be separated into leaf-masses and resolved into cellular modifications. Thus the dicotyledons give, as everyone knows, two seed lobes, or actual seed leaves in germination, as in the case of the pea (fig. 1); whilst the monocotyledons, represented by the

Fig. 3.Fig. 4.

exaggerated section of oat (fig. 2), exhibit the rudimentary forms of future leaves. In the leaf itself, whether we take the curved or divergent venation of endogen (as in figs. 3 and 4), or the netted venation of exogen (as in fig. 5), we can see, through the outer cuticle or skin, the veins ramifying through the inner portion or parenchyma; and botanists all admit these to be modifications

Fig. 5.Fig. 6.

of the cell—the veins in the first instance (fig. 3) running parallel to one another from end to end of the leaf; in the second instance (fig. 4) they are given off from one principal vein or midrib; and in the last case (fig. 5) constitute a complicated net-work or numerous branches and branchlets. The parallel venation will be found in the leaves of lilies, palms, bananas; the nettled, in those of oaks, beeches, &c.^[1] The most remarkable modification of the leaf-type are probably the Indian cups (Sarracenia) of North America, and the still more elegant shapes of the pitcher-plant (Nepenthes) (fig. 6). Sometimes, as in Gloriosa superba, the midrib, which, as shown above, is in fact the principal vein of the leaf, becomes lengthened out into a tendril. The tendrils in the vine proceed from the lateral leaf-buds; those of the passion-flower from the terminal; whilst the solitary thread-like attachment of the cucumber leaf is a sort of stipule, the stipules being leaf-like organs situated on either side of the point at which the leaf is attached to the stem. The pea and vetch having compound leaves, the tendril is formed of the end of the common footstalk, whilst in the vanilla-plant the whole leaf is sometimes elongated into a tendril.

If we notice the arrangement of leaves on the stems of plants, we will find that it proceeds invariably in a spiral—a favourite mode

1. Amonst monocotyledons, Arum, Calla, Lilium giganteum, &c., are considered exceptions to the general rule, from having netted venation; and amongst dicotyledons, Nereum, &c., from having parallel.