. Botany for agricultural students . Botany. 184 STEMS I 'I from the leaf bearing portion of a branch to the regions behind where food manufacture is being abandoned, the following struc- tural features are plainly seen. First, there are the leaf scars, left where the leaves fell away, and interesting because of the way they are formed. (Fig. 160.) As the time approaches for leaves to fall, a cork- like layer, known as the absciss layer, forms across the base of the leaf, severing the direct connections of the leaf with the twig and re- maining as a covering over the scar after the leaf falls.
. Botany for agricultural students . Botany. 184 STEMS I 'I from the leaf bearing portion of a branch to the regions behind where food manufacture is being abandoned, the following struc- tural features are plainly seen. First, there are the leaf scars, left where the leaves fell away, and interesting because of the way they are formed. (Fig. 160.) As the time approaches for leaves to fall, a cork- like layer, known as the absciss layer, forms across the base of the leaf, severing the direct connections of the leaf with the twig and re- maining as a covering over the scar after the leaf falls. The absciss layer closes the open- ings which would otherwise be left by the falling of the leaf, and thereby prevents the entrance of destructive organisms into the twig. It is in connection with leaves which still remain on the tree after the absciss layer is formed that the various autumn colors oc- cur due to changes in the dying leaf tissues. Second, there are the lens-shaped dots, known as lenticels, which, although common on the branches of all woody plants, are espe- cially conspicuous on Fiu. 160. Twig of the branches of the Cherry and Birch. (Fig. 161.) The for- mation of lenticels accompanies the forma- tion of bark. In the young twig, where the protective covering is an epidermis, air is supplied to the tissues beneath through the slit-like openings of the sto- mata; but, as the twig becomes older and bark is formed, the stomata are replaced by lenticels. Lenticels are stomata dis- torted and transformed in structure by the development of bark. Just beneath each stoma, instead of cork, there is formed a loose mass of cells, and this loose mass of cells is pushed up into the opening of the stoma, as shown in Figure 162, rupturing the stoma and surrounding cells and thus the White Walnut, showing leaf soars (a).. Fig. 161.—Twig of Birch, showing Please note that these images are extracted from scanned page images that may have been digitally enhanced fo
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1919
