410 PRIMARY ARRANGEMENT OF TISSUES. 



many-armed or lamellar cavernous parenchyma (comp. p. 211), which from this 

 spongy character has also been called ' spongy parenchyma.' Yet we often find 

 the cells of both layers of irregular form, and differing only in the size of their 

 protrusions, and of the air-cavities thus produced, e. g. in many leaves of Ferns, 

 as Scolopendrium vulgare, Aspidium falcatum, Filix mas, &c. It is superfluous to 

 mention further examples, but we may refer to the flat foliage-leaves of Dicotyledons 

 and Monocotyledons (Lilium bulbiferum, Aroideae, &c.), which are dark-green above, 

 and dull-green below. 



The dense parenchymatous layer is as a rule less thick than the lacunar one, 

 in many leathery leaves scarcely half as thick, e. g. Malpighia macrophylla. The two 

 are usually sharply marked off one from another, yet there are also cases of quite 

 gradual transition. In the leaves of species of Podocarpus, Cunninghamia sinensis, 

 Sequoia sempervirens, Cephalotaxus, and in many Cycadese, as Encephalartos and 

 Zamia integrifolia, the dense, more or less decidedly palisade-like layer is internally 

 bounded by a few layers of loose parenchyma, consisting chiefly of transversely 

 elongated cells containing chlorophyll, which must be distinguished from the trans- 

 verse elements of the border of tracheides (p. 380) ^ 



As mentioned above, the dense parenchymatous layer always corresponds to the surface 

 turned towards the light, the loose layer to the other side. The former is, as a rule, 

 both morphologically and actually the upper surface of the leaf, the latter the lower. 

 These conditions are reversed in the case of the upright leaflets with the edge rolled 

 inwards of Passerina hirsuta'', filiformis, and ericoides, where lacunar parenchyma lies on 

 the densely hairy concave upper side, and remarkably dense palisade-parenchyma on the 

 convex lower side. A similar relation exists, though in a less marked manner, in the 

 leaves of Juniperus communis and nana'. In the flat foliage leaves of Allium ursinum, 

 Alstroemeria, Geitonoplesium, Eustrephus, and also in many Grasses, the morphologically 

 upper side becomes turned downwards, by torsion of the petiole or of the base of the 

 leaf; this side here has the loose parenchyma, while the side which actually faces upwards 

 has the dense tissue *.'.'• 



On the relation of the smaller vascular bundles to the two layers of bifacial leaves, 

 comp. p. 305. 



As has already been indicated, intermediate forms between the main types distinguished 

 are not wanting, and among them the greatest variety in detail prevails. 



As regards the distribution of the two main types according to the forms of leaves and 

 systematic divisions, no general rule, as lar as our present knowledge extends, is to be 

 . added to the few that have been adduced above. The principles brought forward that 

 leaves which are not flat and not horizontally placed always have centric chlorophyll- 

 parenchyma, while the latter is bifacial in horizontal and flat leaves only, hold good in 

 this form for all the groups and families which have come under consideration. They 

 cannot however be extended further, nor does the converse hold good, for among flat 

 and horizontal leaves the greatest differences occur in closely related forms, e. g. bifacial 

 structure in Dianthus barbatus, Statice latifolia, Melaleuca hypericifolia. Eucalyptus pul- 

 verulenta; Allium ursinum, Epidendron ciliare ; centric structure in Dianthus Caryo- 



' See Thomas, Pringsheim's Jahrb. IV. p. 37.— Kraus, ibid. pp. 323, 333, &c. 



^ Caruel, in Nuov. Giorn. hot. Italiano, I. p. 194. Already indicated in De CandoUe, Organo- 

 graphie, I. p. 274. Compare above, p. 49. 



^ Thomas, /. c. p. 39. 



* Treviranus, Physiol. I. p. 445. — Irmisch, Knollen- iind Zvviebelgewachse, p. 4. — A. Braun. 

 Botan. Zcitung. 1870, p. 551. 



