PRIMARY PARENCHYMA. LAMINA. 4II 



phyllus and plumarius, Statice purpurea, Melaleuca linearifolia, Eucalyptus Gunnii ; 

 Allium Nigrum, and the thick-leaved Epiphytic Orchidese, mentioned above at p. 407. 



In addition to the parenchymatous masses described above, there is present in 

 many leaves, and especially in tough ones, a single- or many-layered Hypoderma \ 

 which is, no doubt, always a continuation of the equivalent layer of the stem or petiole. 

 It consists sometimes of elements to be described in the next chapter, sometimes, as in 

 many cases belonging to this category, of thin-walled aqueous parenchyma, the cells 

 of which are in uninterrupted connection with one another and with the epidermis, and 

 in many-layered hypodermata increase in size towards the inside, thus corresponding 

 in all these relations, as well as in the nature of their contents, to the elements of a 

 many-layered epidermis, from which they are only to be distinguished by their origin. 

 Comp. p. 32, and Fig. 44, p. 104. They thus form a strengthening layer to the 

 epidermis. As regards the nature of their walls they often approach the collen- 

 chyma, so that in many cases a sharp distinction from the latter is impracticable, and 

 in this respect also they correspond to many epidermal cells. In the leaves of the 

 Pleurolhallideae, mentioned below, their walls have reticulate or spiral thickenings, 

 in Physosiphon, the Bromeliaceae to be mentioned, and in ^Eschynanthus the lateral 

 walls vertical to the surface of the leaf are folded, with undulations parallel to 

 the latter. 



Many tough Fern-leaves afford examples of this phenomenon, as Polypodium 

 Lingua, Aspidium coriaceum ; many CommeUneae, e.g. species of Tradescantia ; all 

 Scitaminese where the point has been investigated, as Musa, Strelitzia (Fig. 29, 

 p. 85), Heliconia, Canna, Costus sp. ; many Palms, as Chamasrops, Caryota, &c. ; 

 many Grasses, e.g. Arundo Donax ; many Bromeliaceae (Bromelia Caratas, Pholi- 

 dophyllum zonatum, ^chmea, Hechtia, Tillandsia spec), Orchideae of the group 

 Pleurothallideae : Pleurothallis ruscifolia, Lepanthes cochlearifolia, Physosiphon 

 Loddigesii, Octomeria graminifolia, Stelis spec. ; species of Pandanus ; Aroideae 

 (Philodendron cannifolium, Anthurium membranuhferum) ; ^schynanthus spec. ; 

 Veronica speciosa, Lindleyana ; Stenocarpus sinuatus ; Mahonia Fortunei ; Quercus 

 glabra; Ilex aquifolium, ovata, and other species; Rosmarinus officinalis, Nerium 

 Oleander, &c. &c.^ 



The hypodermal layers in question consist of one, of several, or of many strata 

 of cells ; they are as a rule extended uniformly over the whole surface of the leaf, 

 and only interrupted at the stomata — either by a simple intercellular air-cavity 

 (e.g. Fig. 29, Strelitzia), or by chlorophyll-containing parenchymatous cells sur- 

 rounding such a cavity. On the upper surface of the leaf they are usually more 

 bulky than on the lower, or they are exclusively present on the former. In many 

 Bromeliaceae (B. Caratas, Hohenbergia strobilacea), Orchideae, and .^i^schynanthus, 

 they there attain a thickness which amounts to \ or even more than | of the entire 

 thickness of the leaf. In the leaves of many Grasses, as Arundo Donax, the thin- 

 celled hypoderma is limited to narrow longitudinal bands, alternating with the 

 vascular bundles, and covered by large, bladder-like epidermal cells ; on the upper 



* Kraus, Cycadeenfiedern, I.e. — Pfitzer, Pringsheim's Jahrb. VIII. 



"^ Compare Treviranus, Physiol. I. p. 450.— Thomas, Coniferen-blatter, /.r.— Kraus, Plitzer, I.e. 



