436 PRIMARV ARRANGEMENT OF TISSUES. 



rhizome and stem of INIacIeya cordata, similar sacs, sometimes very much elongated, 

 are scattered in large numbers about the perijihery of the ring of vascular bundles, and 

 in the medullary rays. As tlic parts grow old the reddish-yellow colour of the sap dis- 

 appears. The elongated sacs bordering on the fibrous bundles of the bast then acquire 

 thickened walls, like sclerenchymatous fibres. 



5. According to their structure, the latex- and tannin-tubes of many Aroideffl^ belong 

 to this part of our subject. Their main trunks lie in the periphery of the phloem of the 

 vascular bundles, usually two or more together, in collateral bundles situated as a rule 

 symmetrically on each side, in the 'compound' bundles less regularly distributed. In 

 their most perfectly developed form, in Caladium and its allies, Alocasia, Xanthosoma, 

 Syngonium, &c., they constitute thin-walled sacs, about equal to the sieve-tubes in 

 width, following the longitudinal course of the bundles ; at the limiting surfaces of the 

 surrounding parenchymatous cells they send out numerous, pointed or blunt, blind 

 protrusions, which penetrate between the latter. Other protrusions are extended to 

 form longer tubular branches, penetrating between the surrounding tissue-elements, and 

 these also sometimes end blindly, often showing somewhat enlarged ends, while some- 

 times they meet similar branches of neighbouring trunks, and enter into open communi- 



. cation with them. The network of tubes thus formed is extended throughout the 

 parenchyma, not only between the vascular bundles, but also not uncommonly in the 

 parenchymatous cortex, as far as the under side of the epidermis. 



The branches of the network of tubes also run towards the tracheae, attach their ends 

 to the latter, and, as stated above, frequently enter into direct communication with 

 them. 



The contents of these tubes consist of a finely granular fluid, which, according to 

 Trecul, is milky in species of Syngonium and Xanthosoma, in other cases only a little 

 . turbid, and very rich in tannin, so that after the action of iron salts or potassium bichro- 

 mate the net-work of tubes comes out darkly stained. 



Other Aroideae (Richardia africana, Arum vulgare, Dracunculus, Aglaonema simplex, 

 Dieffenbachia Seguine, and species of Philodendron) have no laticiferous tubes, but contain 

 longitudinal rows of elongated cylindrical or prismatic sacs in the periphery of the phloem, 

 with the same arrangement as the trunks of the net-work of tubes described above. 

 They contain the same turbid contents as the tubes described (in Dieffenbachia Seguine 



• according to Trecul they are without tannin), but they are separated by cross-walls 



• and are without lateral anastomoses, only sending out short blind protrusions between 

 . the limiting surfaces of the neighbouring parenchymatous cells. 



A third category — Heteropsis, Lasia, Scindapsus, Monstera, Anthurium, Acorus, &c. — 

 is wholly destitute both of the tubes described, and of sacs. 



That these net-works of tubes arise from the union of originally separate, branched 

 . cells, is shown both by the history of their development, and by comparison with the 

 sac-tubes of the second category. 



6. In the species of the genus Musa, the vascular bundles in the stem, the petiole, the 

 midrib, and the lamina of the leaf (especially also in the fruits) are accompanied by wide 

 laticiferous tubes which are arranged symmetrically, 2-6 around each bundle, and in fact 



. around both phloem and xylem ; usually, however, they are not in direct contact with 

 the bundle, but are separated from it by 1-2 layers of parenchymatous cells. The tubes 

 in the stem and petiole are unbranched, and each consists of a row of cylindrical sacs 

 standing vertically one above another, which are about four times as long as broad, and are 

 united to form a continuous tube by means of a wide round opening in every cross-wall. 

 Round every cross- wall the tube is somewhat constricted, as was well represented even 

 by P. Moldenhawer. The tubes contain large, homogeneous, strongly refractive spheres 



' Karsten, Monatsber. d. Berliner Acad. 1857; Gesammelte Beitrage, p. 253. — Trecul, Van 

 Tieghem, Ilanstei.i, /. r. 



