424 PRIMARY ARRANGEMENT OF TISSUES. 



Isolated fibres, sometimes ramified, sometimes unbranched, occur as a widely- 

 distributed and characteristic phenomenon in the parenchyma of the, cortex and 

 leaves of many Gymnosperms, and to this mode of occurrence, as well as to that 

 described in the case of the Nymphseaceae and Aroideae, their appearance in a 

 number of tough Dicotyledonous leaves is related. 



Many of the phenomena belonging to this category, and the literature referring 

 to them, have already been mentioned in Sect. 30 (p. 130), to which reference must 

 therefore be made. 



Among the Gymnosperms, many CycaJex (Dion, Ceratozamia, Encephalartos, &c.), 

 and several Coniferx (e.g. Cunninghamia, Fig. 183, p. 380), show longitudinal unbranched 

 fibres, occurring isolated or in small groups, in the parenchyma of the petiole and leaf. 

 The same applies to the cortex of Ephedra. Stellately-branched fibres lie scattered in 

 the chlorophyll-parenchyma in Sciadopitys, Dammara, and Araucaria imbricata. The 

 latter recall the fibres, differing from those of the hypoderma, by which the entire paren- 

 chyma ofWelwitschia, even including that of the floral organs, is abundantly permeated, 

 the resemblance consisting especially in the presence of great numbers of crystals of 

 calcium oxalate deposited on their surface: they are thick and short spindle-shaped 

 elements, with short protruding branches here and there at their pointed ends, and with' 

 a very thick, much stratified, lignified wall (comp. Fig. 55, p. 132, and Fig. 187, p. 408). 

 In the stem these fibres are directed without order towards different sides. In the leaves, 

 where they are on the average somewhat narrower than those of the stem, they lie as a 

 rule, not without exception, in the middle lamella, parallel to the surface of the leaf, with 

 their longitudinal axis sometimes directed longitudinally, sometimes transversely or 

 obliquely with reference to that of the leaf ; they usually stand about at right angles to 

 the surface of the leaf, on both sides of the middle lamella, in the chlorophyll tissue tra- 

 versed by fibrous bundles ; they reach the middle lamella with one end, and the inner 

 surface of the epidermis with the other, and often have a hook-like bend at the latter, or 

 are even wedged in between the inner parts of the epidermal cells. In the parenchyma of 

 the third genus of Gnetaceae, Gnetum, at least in the species investigated, sclerenchyma- 

 tous fibrous cells are no less abundant than in Ephedra and Welwitschia ; in Gnetum 

 Gnemon they are present in the entire parenchyma of the external cortex, here running 

 longitudinally, and branching rarely or not at all, also in the pith of the nodes, and in the 

 leaves near their surfaces, especially the upper, to which they are nearly parallels 

 in Gnetum Thoa they occur in the same way in the outer cortex, but especially in the 

 pith of the nodes and in the leaves ; in the regions last mentioned they are abundantly 

 and variously ramified, in the leaf their size is very unequal, sometimes very considerable, 

 Comp. p. 130. 



The leathery leaves of all these Gyinnosperms are thus strengthened by a complicated 

 sclerenchymatous frame-work. Among Dicotyledons the leaves of Camellia and Fagrxa^ 

 are characterised by numerous fibres scattered in the parenchyma, which are abundantly 

 and irregularly branched (Fig. 53, p. 130) ; this also applies to the leaf of species of Olea, 

 In Olea europaea the fibres are very irregular in their ramification and direction, sending 

 out branchies on all sides as far as the under surface of the epidermis ; in O. fragrans 

 ' they extend, usually without branching, right across the whole leaf at right angles to 

 the surface, and branch in a more or less pedate manner at the upper and lower epider-' 

 mis, so that they act as columns, connecting the two epidermal layers together.' (Thomas.) 

 In the Proteaceae mentioned at p. 1 30, rod-shaped, more or less ramified sclerenchymatous 

 fibres stand between the palisade-like chlorophyll-cells, at right angles to the inner surface 

 of the epidermis. They are as high as, or somewhat higher than the parenchymatous 

 layer on either side which contains the chlorophyll, and with their usually branched ends 

 they adhere on one side to the epidermis, while on the other they are attached to or 

 wedged into the middle layer of the leaf, which in the thick-leaved species is destitute of 



