SCLERENCHFMA AND SCLEROTIC CELLS. 423 



mentioned above as examples of the contrary. Among the tough stems of Mono- 

 cotyledons, the Bambuseoe investigated show the same behaviour. In many stems 

 of Monocotyledons the sclerenchymatous strands occur chiefly, though not ex- 

 clusively, in company with the vascular bundles ; this is the case in the thick stems 

 of Zea, Saccharum, &c., Palms, and Pandaneae ; also in the stems and petioles of 

 Aroideae, as Colocasia, Arum, and many others. It is the rule in IMonocotyledonous 

 stems, and petioles resembUng them in structure, that both the relative and absolute 

 thickness of the strands accompanying the vascular bundles, as well as the thickness 

 of the walls of their elements, increase as they approach nearer to the periphery of 

 the bundle-cylinder. In the Aroideae mentioned, the bundles, with the exception of 

 the outermost circles, are without a fibrous covering. In most Palm stems, the 

 periphery of the bundle-cylinder, which is surrounded by a narrow cortex, is formed 

 of massive fibrous bundles, separated by narrow bands of parenchyma ; on the inner 

 side of each of them a small vascular bundle is attached or inserted ; this region 

 therefore consists chiefly of firm masses of sclerenchyma, while the bundles in the 

 interior of the stem, as follows from their general course (p. 262), stand farther apart, 

 and have in every respect a weaker fibrous investment. 



Finally, it scarcely needs to be especially stated, that in the case of vascular 

 bundles which are accompanied by fibrous strands and which gradually become 

 longitudinally united a union of the fibrous strands also takes place. If the latter 

 happens earlier than the union of the vascular bundles themselves, the latter, as seen 

 in cross-section, appear inserted, two or more together, in one fibrous strand, as is 

 conspicuously evident in the periphery of Palm-stems, and especially in the stems of 

 the Pandaneae. 



Lastly, we must here once more call attention to the fact that the fibrous 

 strands are often derived from collenchymatous elements. Those strands, or those 

 sections of them, which belong to parts characterised by long-continued capacity for 

 growth and curvatures due to growth, show collenchymatous properties as long as 

 this capacity is maintained, or they show an intermediate character between 

 sclerenchyma and collenchyma ; e. g. the base of the sheaths of leaves in the 

 Grasses (Fig. 151), and the above-mentioned stems of Aroideae. 



Sect. 126. In certain relatively rare cases, isolated sclerenchymatous fibres, or 

 fibres united through part of their course to form small bundles, occur in the 

 parenchyma, external to, and usually side by side with, the strands and layers 

 described. To this category belong, in the first place, those branched elements 

 projecting into the air-spaces, which under the name of internal hairs have already 

 been minutely described above (Sect. 53, p. 221) in the case of the Nymphseaceoe, 

 Limnanthemum, Rhizophorese, and many Aroideae. Elements more or less similar 

 to those mentioned occur elsewhere fixed in dense parenchyma. As isolated cases 

 of this sort may shortly be mentioned the unbranched fibres in the cortex of the 

 root of Chamaedorea elegans, already dealt with at p. 129; also the branched fibres 

 in the pith of Carissa arduina\ and the often branched fibres, 6-14™™ in length, 

 which TrecuP found in the cortex of Euphorbia rhipsaloides, and in the pith and 

 cortex of E. xylophylloides. 



' A. Gris, /.c, compare y. 403. = Comples Kendus, LX. p. 1341). 



