9a 



MORPHOLOGY OF TISSUES. 



the formation of periderm in one-year-old branches as long as the cortex is still 

 ' covered with uninjured epidermis; and are visible as roundish flakes. At the end of 

 the first or in the following summer, the epidermis splits above the lenticel in the 

 direction of its length ; it becomes changed into a more or less projecting wart, 

 which is often divided by a central furrow into two lip-shaped rolls ; their upper 

 surface is generally brown, their substance to a certain depth dry, brittle, and cork-like. 

 With the further increase in thickness of the branch, the lenticels become extended in 

 breadth and present transverse striae ; when afterwards cork or bark is formed, the 

 splitting of the cortex in the lenticels commences, and they become indistinguishable 

 (as in the silver poplar, apple and birch) ; by the scaling off of the bark they are of course 

 removed. According to Unger, the lenticels arise only at those places of the cortex where 

 stomata occur in the epidermis ; according to JMohl the inner cortical parenchyma pro- 

 jects in a wart-like manner through the outer, and forms there a cork -tissue, which, on 

 the formation of periderm, coalesces with the cork of the periderm ; as occurs also, for 

 example, in young potato-tubers. The formation of cork on the lenticel lasts for a series 

 of years, until the cortex which afterv/ards grows from within dies off on the outside, 

 the periderm or bark-formations becoming interposed between the lenticels and the 

 living part of the cortex. In many trees (as Crat^gus, Pyrus, Salix, Populus), where 

 the formation of periderm begins from single points, and then becomes further extended 

 in breadth, the lenticels are, according to Mohl, the points of departure. 



Sect. i6. The Fibro -vascular Bundles \ — The tissue of the higher 

 Cryptogams and of Phanerogams is traversed by string-like masses of 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 strong masses, retaining, however, in- 

 ternally the corresponding structure. These 

 are the Fibro-vascular Bundles. Very often 

 they can be completely isolated with ease 

 from the rest of the tissue of the plant. If, for 

 instance, the leaf- stalk of Plantago major is 

 broken across, they hang out from the paren- 

 chyma as tolerably thick, flexible, elastic threads. 

 In Pteris aqiiilina it is possible, by scraping 

 off the mucilaginous parenchyma after removing 

 the hard skin of the underground stem, to ex- 

 pose them as strap-shaped or filiform very 

 firm light yellowish bands (Fig. 80). From older 

 foliage-leaves of trees, dry pericarps (as Da- 

 tura), stems of Cactus, tSic, the fibro-vascular 

 bundles are left, through the decay of the par- 

 enchyma which surrounds them, as a skeleton 



Fig. Zo.— Pteris aqidlina. A transverse section of 

 the underground stem (natural size) ; r brown hard 

 epidermal tissue ; / soft mucilaginous parenchyma, 

 rich in starch ; pr dark-walled sclerenchyma, forming 

 two broad bands penetrating the stem ; ag fibro- 

 vascular bundles running outside these bands of 

 sclerenchyma ; ig others running within them. 

 B the fibro-vascular bundle represented in A, iso- 

 lated by scraping off the parenchyma ; it shows divi- 

 sions and anastomoses ; the dotted lines u show the 

 outline of the stem st, its forked branches st' and st", 

 and a leaf-stalk b. 



^ H. von Mohl, Vermischte Schriften, pp. 108, 129, 195, 268, 272, 285, 1845,— Ditto, Bot. Zeitg. 

 p. 873, 1855. — Schacht, Lehrb. der Anat. u. Phys. der Gewuchse, pp. 216, 307-354, 1856. — Nageli, 

 Beitrilge zur wiss. Bot. Leipzig 1858. Heft i.— Sanio, Bot. Zeitg. no. 12 et seq. 1863.— N.lgeli, Das 

 Dickenwachsthum des Stammes u. die Anordnung der Gefassstriinge bei den Sapindaceen, Miinchen 

 1864. — Rauwenhoff, Archives Neerlandaises, vol. V, 1870. (Caractere et formation du liege dans les 

 dicotyledons.) 



