. Carnegie Institution of Washington publication. . 72 THE WATERLILIES. The lower epidermis (Fig. 29, a) has been frequently referred to in previous pages. It is a perfectly continuous layer, to mm. thick, quite smooth on the outer surface, and consisting for the most part of polygonal cells, many of which are about twice as long as broad ( X mm.). In these is located the pigment which colors the under side of the leaf. More or less plentifully interspersed with them are thickened circular cells (suberized, according to Schilling, 1894) representing the bases of hairs. Fr
. Carnegie Institution of Washington publication. . 72 THE WATERLILIES. The lower epidermis (Fig. 29, a) has been frequently referred to in previous pages. It is a perfectly continuous layer, to mm. thick, quite smooth on the outer surface, and consisting for the most part of polygonal cells, many of which are about twice as long as broad ( X mm.). In these is located the pigment which colors the under side of the leaf. More or less plentifully interspersed with them are thickened circular cells (suberized, according to Schilling, 1894) representing the bases of hairs. Frequently the other epidermal cells are arranged around these in a radial manner. This caused Barthelemy (1874) to speak of them as pseudostomata, and he thought he could see in the center of some of them a small perforation. His opinion lacks confirmation. The circular cells are about mm. l" across in , mexicana, amazonum, }^J^ odorata, o. rosca, and tuberosa ; to mm. in N. zanzibariensis X and alba candidissima, and to mm. in N. tetragona. Their relative number is shown by the following figures, represent- FIO. ^.-Development of mucilage hah-otjv. ing the number visible in a single field alba; 1-5, successive stages. (After Schilling.) . ... of my microscope (Leitz, oc. 2, obj. 7) : N. /lava 32, mexicana 19-20, amazonum 52, tetragona 21, odorata 36, o. rosea 34, tuberosa 28, rubra 46. In the last-named species permanent hairs stand on many of these basal cells. Hofmeister in 1868 mentioned N. alba in company with Fagus, Salix, Ouercus, &c., as having hairy leaves only in the bud. From what has already been said concerning the occurrence of basal cells of hairs in the epidermis of the petioles and lower leaf-surface, it will be seen that hair structures always occur on these parts at some time in their history ; the main ribs of the leaves and their upper surface are alone totally devoid of them. On a leaf rudiment of N. flava a trifl
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