426 PRIMARV ARRANGEMENT OF TISSUES. 



massive branch into the thorn-leaf, which in the broad base of the thorn forms as a 

 thick plate the larger lower lialf ; the narrower upper half is thin- walled parenchyma, 

 in which the vascular bundles lie. As the thorn tapers off the hypodermal scleren- 

 chyma increases in relative extent, at the cost of the other tissue, in such a manner 

 that in the cylindrical upper part there are only feeble vascular bundles in the 

 middle, enclosed in scanty parenchyma, and completely surrounded by the scleren- 

 chvmatous mass, while the apex is formed of the latter and of the epidermis 

 exclusively. The ends of the petiolar, stipular, and branch thorns of Astragalus 

 aristatus, Ilalimodendron, Robinia, ]Maclura, Crataegus, and many thorny teeth of 

 leaves, show a similar structure. For further details compare the works cited at 

 p. -,7, by Delbrouck, Uhlworm, Suckow, &c.; v. jMohl, Palm. Struct, p. 7 ; Lestiboudois, 

 Comptes rendus, Tom. 61, pp. 1034, 1093. 



Sect. 129. The sclerotic elements of the Ferns and Hydropterideae ^ are, as ex- 

 plained in Sects. 26 and 28, sometimes fibrous cells containing starch, sometimes 

 specific sclerenchymatous elements ; the division of labour between the two forms 

 is not however strictly carried out, and a sharp severance of the two is not 

 possible. Their arrangement corresponds on the whole to the rules stated 

 for the Phanerogams in preceding paragraphs ; even the peculiarities to which 

 attention is to be called can generally be brought under these rules as special 



cases. 



Sclerotic hypodermal masses of tissue are absent in the stems or rhizomes 



of many Ferns, the hypodermal zone being only distinguished from the internal 



parenchyma, into which it gradually passes over, by the 

 closer connection, smaller width, and somewhat thicker walls 

 of its cells ; e. g. Polypodium vulgare, pustulatum, Davallia 

 elegans, Acrostichum vexillare, Angiopteris ; Marsiliace^e 

 with a many-layered dense hypodermal zone of parenchyma, 

 which passes over internally into the inner zone, which 

 riG. T8s.-os.iiunda regaiis; is travcrscd by a circle of wide air-passages. ]Many stems, 



cross-section of avigorous stem, , • ii .1 ii • i 1 L ii u j 



seen from above, i. e. from the and cspecially thc thicRcr oucs, havc on the other hand a 



apex of the stem, niaj;nified .... , 1 1 ,• • i- f 1 



about 20. i lowest leaf-trace distuict hypodcmial sclcrotic zone, consistmg of several or 



bundle; a root-bundle given off , _ . , ji •» j 1 i u • 1_ • 



fron.this.sseenpassingthrough mauy laycrs of unuitcrruptedly united elements, wfncii ni 



the true Ferns always have brown membranes; in most 

 cases this zone does not border directly on the epidermis, but is separated from it 

 by some layers of thin-walled parenchyma. This is the case, for example, in the 

 Cyatheaceae, Polypodium Lingua, Platycerium, Davallia pyxidata, &c. The scleren- 

 chymatous ring, which lies in the cortex of the thin stem of Hymenophyllese, may 

 just as well be mentioned here as among the instances of sclerenchyma accom- 

 panying the vascular bundles, to be brought forward later. The same applies to the 

 dark-brown mass of sclerotic elements containing starch, which in Osmunda regalis 

 and Todea hymenophylloides forms the principal part of the stem, and which is every- 

 where sharply marked off from the relatively small colourless tracts of parenchyma 

 containing the vascular bundles. Comp. pp. 346 and 279, and Fig. 188. Sclerotic 

 annular layers bordering directly on the epidermis occur more rarely, e. g. in the 



' Compare the literature cited above, §§ 73-S7, on tlic structure of llic Fern-stem. 



