THE FIBRO-VASCULAR BUNDLES. 



109 



tissue, which in some cases develop by increase in thickness in such a manner 

 that they lose externally the form of strings and present that of large masses, 

 retaining, however, internally their characteristic structure. These are the Vascular 

 or Fihro-vascular Bundles. Very often they can be completely isolated with ease 

 from the rest of the tissue of the plant. If, for instance, the petiole of Plantago 

 major is broken across, they hang out from the parenchyma as tolerably thick, 

 extensible, elastic threads. In Pteris aquilina it is possible, by scraping off the 

 mucilaginous parenchyma, after removing the hard epidermal tissue of the under- 

 ground stem, to expose them as strap-shaped or filiform very firm light yellowish 

 bands (Fig. 91), In older leaves of trees, dry pericarps (as Datura), stems of 

 Cactus, &c., the fibro -vascular bundles are left, through the decay of the parenchyma 

 which surrounds them, as a skeleton retaining more or less the original form. 

 Beautiful and instructive skeletons of this nature 

 are afforded by the stems of Tree-ferns, Dra- 

 ccEfia, Yucca, maize, &c., when their parenchyma 

 has been destroyed by gradual decay, and 

 only the epidermal tissue and the firm bundles 

 in the interior remain ; and the student would 

 do well in any case to make for himself pre- 

 parations of this kind, or to examine them in 

 collections ; they are extremely useful for a clear 

 comprehension of structure. This is, however, 

 the case only with lignified fibro-vascular bundles 

 which run isolated between soft parenchyma ; 

 in some plants, on the contrary, the tissue of 

 die bundles is even softer and more delicate 

 than that which surrounds them {e.g. Cerato- 

 phyllwu, Myriophyllum, Hydrillese, and other 

 water-plants) ; and in these cases they cannot of 

 course be isolated. But in the older lignified 

 stems and roots of Conifers and Dicotyledons, 

 the fibro-vascular bundles are so densely crowd- 

 ed, and so extended by the further development 

 of their tissue, that at last very little or even 



nothing is left of the original fundamental tissue which separated them, and such 

 stems consist almost entirely of fibro-vascular masses. 



Each separate fibro-vascular bundle consists, when it is sufficiently developed, of 

 several different forms of tissue, and must therefore itself be considered as a tissue- 

 system ; but different bundles, often in very large number, unite in most plants to 

 form a system of a higher order. At present however we shall consider only the 

 separate bundle. 



The fibro-vascular bundle consists at first of similar cells fitting together 

 without intercellular spaces ' ; this form of tissue in the young undifferentiated 



FIG. ^x.— Pteris aqiiiliiia. A transverse section 

 of the underground stem (natural size) ; r lirown hard 

 epidermal tissue ; / soft mucilag nous parenchyma 

 containing starch ; pr dark-walled sclerenchyma, form- 

 ing two broad bands traversing the stem ; a^ fibro- 

 vascular bundles running outside these bands of 

 sclerenchyma ; ig others running within them. B the 

 fibro-vascular bundle represented in A, isolated by 

 scraping off the parenchyma; it exhibits divisions 

 and anastomoses; the dotted lines it sliow the outluie 

 of the stem s , of its branches s^ -and st" , and of a 

 petiole b. 



^ The young cells of the fibro-vascular masses are not always elongated and prosenchymatous ; 

 in the roots of maize the young vascular ctlls which no longer divide, as well as the adjoining ones, 

 are tabular or cubical. 



