. Libro di testo di botanica, caratteristiche morfologiche e fisiologiche. La botanica. Le foglie e cuscinetto-assi, -155 appartenenti al dermatogen, come in Utricularia secondo Pringsheim. Ma in crittogame il dermatogen diventa differenziati solo dopo la formazione della foglia; e quindi i peli sono sempre a una distanza maggiore dal vertice rispetto alle foglie più giovani (Fig. 116); le cellule superficiali dello stelo, che in crittogame diventa la cellula apicale di una nuova foglia, non è una cellula epidermica, poiché la sua origine risale a lungo prima che la differenziazione del tessuto nell'epidermide e periblem. (5) il tessuto

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. Text-book of botany, morphological and physiological. Botany. LEAVES AND LEAF-BEARING AXES, -155 belonging to the dermatogen, as in Utricularia according to Pringsheim. But in Cryptogams the dermatogen becomes differentiated only after the formation of the leaf; and hence the hairs are always at a greater distance from the apex than the youngest leaves (Fig. 116); the superficial cell of the stem, which in Cryptogams becomes the apical cell of a new leaf, is not an epidermal cell, since its origin dates long before the differentiation of the tissue into epidermis and periblem. (5) The Tissue of the mature Leaf is continuous in its formation with that of the Stem. It is impossible, histologically, to find a boundary line between the stem and the base of the leaf, although such a boundary line must be assumed theo- retically. If the surface of the stem is imagined to be continued through the base of the leaf, the transverse section thus caused is called the Insertion of the Leaf The continuity of the tissue is especially observable in vascular plants, where the well-developed leaves1 consist, like the stem, of epidermal and fundamental tissues and fibro-vascular bundles. The * cortical layers of the stem bend out with- out interruption into the leaf, and consti- tute its fundamental tissue; in the same manner the epidermis passes over from the stem into the leaf; the fibro-vascular bundles of the leaves have, in Phanero- gams and many Cryptogams, the appear- ance of being the upper ends of the ' com- mon ' bundles which ascend in the stem (Fig. 119); and where this is not the case, as in Lyeopodiaceae, the basal por- tions of the foliar bundles and the fibro- vascular mass of the stem are nevertheless in continuity. The main cause of the continuity of tissue between stem and leaf is that the leaf arises from the cone of growth of the stem, where it still consists entirely of primary meristem; in vascular plants the young leaf appears as a luxuriant development of it

Parole chiave associate all'immagine

• bookcentury1800
• bookdecade1880
• booksubjectbotany
• bookyear1882