122 



MORPHOLOGY OF TISSUES. 



lamina; in the petiole they are generally surrounded by a thin-walled parenchymatous 

 fundamental tissue with wide elongated cells; this also forms sheath-like envelopes 

 around the stronger bundles of the lamina, which are conspicuous on its under-side 

 as the Feins ; but the finer branches, and the finest of all, run through the so- 

 called Mesophyllf i. e. a peculiar form of the fundamental tissue distinguished 



by containing chlorophyll. Not 

 unfrequently single cells of 

 the fundamental tissue of the 

 lamina assume the form of idio- 

 blasts, e.g. the larger stellate 

 cells in the leaf of Camellia ja- 

 ponica, the erect rod-like cells 

 upon which the stomata of the 

 leaves of Hakea are, as it were, 

 supported. All these forms of 

 tissue are enveloped by the 

 epidermis, and frequently also 

 by hypodermal tissue. In the 

 carpels of Phanerogams there 

 occurs commonly a more mani- 

 fold difl'erentiation of the funda- 

 mental tissue ; I will instance only 

 the formation of the so-called 

 ' stones ' of Amygdalese. The 

 stone is the inner layer of the 

 fundamental tissue of the same 

 foliar structure of which the outer 

 layers form the succulent flesh of 

 the fruit ; the former is scleren- 

 chymatous, the latter parenchy- 

 matous and succulent, both being 

 traversed by fibro-vascular bun- 

 dles. Equally clear is the struc- 

 ture in the stems of Ferns, among 

 which Tree-ferns and Ptei-ij 

 aquilina are of special interest, 

 because the fundamental tissue 

 occurs in them in two quite 

 different forms. Its preponder- 

 ating mass consists, e. g. in Pteris 

 aquilina (Fig. 91, p. 109) of a 

 thin-walled colourless mucila- 

 ginous parenchyma, and con- 

 taining starch in winter ; parallel 

 to the fibro-vascular bundles 

 there are also threads or 

 bands of prosenchymatous thick- 

 walled dark brown scleren- 

 chyma; these have nothing in 

 common with the fibro-vascular bundles, but are only a peculiar form of the funda- 

 mental tissue, a prosenchymatous form of which often occurs also elsewhere in 

 Cryptogams. The tendency to a prosenchymatous development of the cells of the 

 fundamental tissue is also well seen in the stems of Lycopodiaceae. In Selaginella 

 denticulata (Fig. .100 A) the axial fibro-vascular bundle is surrounded by a very 



FIG. 100. — A transverse section of the stem oi Sela^ineUa dentimlata; the 

 fibro-vascular bundle is not yet fully developed ; the vessels are already ligiii- 

 fied on both sides, but not yet in the centre; / air-conducting intercellular 

 spaces in the parenchyma enveloping the bundle ; towards b is the tissue 

 corresponding to the bundle which bends outwards to the leaf. B transverse 

 section of the mature stem oi Lycopodium Chantacyparisstis, the axial cylin- 

 der consists of densely crowded and coalescent fibro-vascular bundles; the 

 four xylem-portions are quite separated, forming four bands on the transverse 

 section, between and round which are found the narrower cells of the phloem. 

 The phloem portions of the four bundles have coalesced ; between each pair 

 of xylem-bundles is seen a row of wider cells, the latticed cells or sieve-tubes ; 

 the narrow cells lying on the right and left edge of each xylem-portion are 

 spiral vessels (also in A). In the thick-walled prosenchymatous fundamental 

 tissue which envelopes the axial cylinder is seen the dark transverse section 

 of a slender fibro-vascular bundle which bends outwards to a leaf; it consists 

 almost exclusively of long spiral vessels (x about 90). 



