PRIMARY PARENCHYMA. LAMINA. 409 



being inserted in a lamella of round-celled parenchyma containing chlorophyll; 

 these lamellae unite the chlorophyll-layers of the two surfaces of the leaf, and stand 

 at right angles to the latter ; e. g. Hemerocallis fulva, Narcissus pseudonarcissus, 

 Hyacinthus orientalis, Ornithogalum umbellatum, Phormium tenax, many Irideae, 

 Bambusese ^, &c. Secondly, in the leaves of many Monocotyledons, narrow-leaved 

 Eryngia, Lobelia Dortmanna, &c., the middle layer is soon destroyed to form lysi- 

 genetic air-passages. In the round-leaved species of Allium and Asphodelus, in 

 which the middle of the leaf is destitute of vascular bundles, a wide tubular cavity 

 traversing the leaf arises in this way; in leaves where the vascular bundles are in- 

 serted in persistent plates -of parenchyma, there are several passages alternating with 

 the latter ; they are numerous in the Narcissus mentioned above, and in Grasses, 

 Cyperacese, Sparganium, Typha, Pandanus, and Eryngia; in Lobelia Dortmanna 

 there is one on each side of a median persistent plate. 



In tQUgh leaves of Monocotyledons, the more or less complete interruption of 

 the chlorophyll-parenchyma by longitudinal strands of fibrous sclerenchyma, usually 

 containing the longitudinal vascular bundles, is also of frequent occurrence, as 

 will be described in the following chapter ; and there are many other special 

 phenomena of alternately dissimilar longitudinal bands, which respectively contain, 

 or are destitute of bundles, e. g. in the leaves of Grasses, large Bromeliacese, &c., 

 the detailed description of which in this place would lead us too far. 



With regard to the chlorophyll-parenchyma are to be mentioned, as examples of 

 the palisade form, the leaves of the Myrtacese mentioned, Proteace», species of 

 Statice, Welwitschia, stem of Casuarina, Salicornia herbacea, Colletia hoitida, 

 Scirpus Holoschoenus, lacustris, palustris, Papyrus, Juncus effusus, &c. The leaves 

 of many Monocotyledons, Mesembryanthema, the stems of Bossiaea, Miihlenbeckia 

 platyclados, Cacteae, Juncus glaucus, &c., may be mentioned as affording examples 

 of the round-celled form. 



Cells elongated in the transverse direction, i. e. parallel to the surface, are 

 peculiar to the sword-shaped leaves of many Irideae, as Iris Germanica, Gladiolus 

 imbricalus, and especially Tritonia deusta. In species of Pinus and Cedrus the cells 

 of the chlorophyll-parenchyma are characterised by the tabular polyhedral form, 

 and the infolding of the walls, mentioned above at p. 118. (See Figs. 11, p. 35, 

 and 27, p. 78.) 



2. To the second type belong only flat horizontal leaves, and in fact the 

 majority of them. It is characterised by the fact that the chlorophyll-parenchyma 

 forms the entire substance of the leaf, and is severed into two different layers, each 

 of which corresponds to one surface of the leaf. It may accordingly be termed the 

 two-sided, the bifacial type. The difference between the two horizontal layers may 

 be generally expressed by the statement, that the one, that namely belonging to the 

 surface which is turned towards the light, is denser than the other, being furnished 

 with less wide interstices, and therefore appearing of a darker green, even when the 

 amount of chlorophyll in each individual cell is the same. 



The usual form is such that the denser layer consists, according to the thickness 

 of the leaf, of one or more layers of palisade-cells, the other layer consisting of 



' Kareltschikoff, /. <r. ; compare p. 118. 



