210 INTERCELLULAR SPACES. 



their middle part intercalated in the plate of cells, while their ends extend over it, the 

 one upwards, the other downwards. 



—^ As examples of all these conditions in sacs with clustered crystals may be cited 

 Myriophyllum, Trapa, Nelunibium : in raphide-sacs, Pontederia ^, Scitaminese, Phily- 

 drum -, Colocasia odora : in both forms together, many Aroideae, as Colocasia anti- 

 quorum, Caladium nymphceifolium ^, Pistia. 



In the diaphragms of the petiole of Pontederia (P. cordata and crassipes) there 

 are found, besides the raphide sacs, others, of spindle- like shape, with their longer 

 axis placed at right angles to the diaphragm, and with their middle intercalated in it, 

 so that the ends project upwards and downwards into the space : each of these 

 contains a single spear-like crystal sharply pointed at both ends. This, together 

 with the sac which contains it, attains a length, especially in P. cordata, of more 

 than three times the thickness of the diaphragm. Finally, the membrane of the sac 

 covering the ends of the crystal ceases to be apparent, so that the latter seems to 

 protrude freely into the air-space *. 



Sect. 53. The walls of many large lacunae and air-passages are characterised by 

 projecting cells or portions of cells, which, from. their form, are termed hai'rs. These 

 may be divided into two categories, namely^ glandular hairs ; and non-glandular, 

 usually firm hairs like sclerenchymatous fibres. The only forms of the first category 

 are those glandular hairs, first noticed briefly by Mettenius ^ and described later by 

 Schacht", in the air-cavities of the rhizome and base of the petiole of Aspidium Filix 

 mas. One or more unicellular capitate hairs, which are seated singly on adjoining 

 starch-containing parenchymatous cells, project into the larger cavities of this plant : 

 they arise originally as daughter-cells of the latter, or as outgrowths of them. 

 The small thin cylindrical stalk widens out into a large pear-shaped head, and this 

 secretes on its surface, as far as the limits of the stalk, a firm, greenish, brilliant, thick 

 layer of resin. The mode of its secretion and the structure of the whole hair are 

 the same as above described for the glandular hairs (p. 88) ; glandular hairs, which 

 closely resemble these intercellular hairs, occur here and there on the surface of the 

 bases of the petioles of the male fern, as teeth of the base of the Palese. In the 

 base of the petiole of Aspidium spinulosum — and probably also of other allied species 

 — such internal, intercellular glandular hairs are found, though in less numbers than 

 in Filix-mas. 



Intercellular hairs of the second category occur especially in the air-passages of 

 such plants as have no diaphragms : Pilularia, Nymphaeaceae, Aroideae, Rhizophora, 

 also Limnanlhemum. As may be judged from their structure and arrangement, 

 they, like the diaphragms, serve as a support. Russow '^ found in the air-passages of 

 the root of Pilularia globulifera hairs rolled up like watch-springs, with thin mem- 

 branes, studded with fine external warts : their arrangement is as follows. The inner 

 cortex contains 12 air-passages separated by radial lateral walls one layer of cells in 

 thickness; 6 broader ones alternate with 6 narrower ones. Single cells of the lateral 



1 Meyen, Phytotomie, Taf. V. 



^ S. F. Hoffmann, Linnxa, XII. p. 683. ^ Meyen, I.e. Tab. XII. 



* Compare Meyen, Phytotomie, Taf. V. — Duval-Jouve, I.e. p. 166. 



* Fil. horti. Lips. p. 92. * Pringsheim's Jahib. III. p. 352. 

 ■^ Ver<jl. Unters. p. 22. 



