^oS PRIMARF ARRANGEMENT OF TISSUES. 



are succulent and leathery, a middle layer is present, similar to the entire organ in 

 form, which fills up the internal space, and is enclosed by the chlorophyll- 

 parenchyma as by a cortex; in many species of Aloe (A. tesselata, cuspidata, 

 .atrovirens, &c.) it breaks through the latter, as it were, in places, so as to reach 

 the epidermis ^ It consists, as a rule, of relatively large, colourless cells, destitute of 

 chlorophyll, which essentially contain water or sap ; in succulent plants, e. g. many 

 species of Aloe (A. soccotrina, plicatilis, arborescens), in other Monocotyledons and 

 in Mesembryanthemum, they contain abundant thin mucilage (comp. p. ii6); in 

 sappy leaves they have soft walls, in tough leathery leaves thicker pitted walls. On 

 the course of the vascular bundles within, or at the boundary of the middle layer, 

 comp. sect. 92, p. 305, and sect. 112, p. 378. 



Examples of this structure ^ besides those already mentioned, are afforded by 



/■ J- 



Fig. 187.— Cross-section through the leal of Welwitschia mirabilis {40). e epidermis o{ the upper, « of the 

 lower surface ; in their depressions are stomata. g trunks of vascular bundles, £' small branch bundle.; on the 

 trunk marked ^ a similar one is shown branching off. s thick sclerenchymatous fibres containing crystals. 

 Hooker's spicular cells ; yhypodennal and internal strands of long sclerenchymatous fibres ; c — c delicate 

 palisade-parenchyma, containing chlorophyll. Between c and c is the wide-raeshed tissue of the middle layer. 

 Compare Fig. 53, p. 132, Fig. 145, p. 303, and Fig. 157, p. 33s. 



the foliage-leaves of many Coniferae, e. g. species of Podocarpus, Araucaria with 

 iBat leaves, Pinus (Fig. 185, p. 381), the leaf of Welwitschia (Fig. 187), the leaves of 

 Myrtaceae (Callistemon, Eucalyptus Gunnii, Melaleuca tetragona, linearifolia), Pro- 

 teaceae (Hakea spec), Statice monopetala, purpurea; the phyllodes of Oxalis 

 fruticosa and many Acaciae, &c. ; lastly, numerous leaves of Monocotyledons. 



Among the special phenomena, which are extremely various in different species, 

 we have first to mention, with reference to the middle layer, that in many flat leaves 

 of Monocotyledons this is divided into as many longitudinal bands as there are 

 longitudinal bundles running through the leaf (p. 301), each of these bundles 



' Pfitzer, in Pringsheiiri's Jahrb. VIII. p. 64, fig. 20, 



' [Compare Briosi, Contribuzione alia Aiiat. d. foglie. J^e/. Bot. Centralbl. Bd. 11, 1882, p. 55.] 



