2IO INTERCELLULAR SPACES. 



same structure as that in tlie reservoirs of the leaf, wliich does not completely fill the 

 cavity, being surrounded, while young, by colourless granular contents (protoplasm ?). 

 In the roots of Ardisia I did not find the red secretion. 



(g) JNIany, though not by any means all the species of Oxalis from the Cape and 

 America have, on the under surface of the leaf, and running towards the margin, rather 

 prominent reddish bands, which are mentioned in descriptions as glands or wales. In the 

 species — not exactly defined — which I have investigated, these bands are reservoirs quite 

 similar in the colour, consistency, and radiate structure of the secretory mass, and also in 

 the structure of the surrounding tissue to those of Lysimachia punctata and Ardisia. 

 They lie in the chlorophyll-parenchyma, and are separated by but one layer of its cells 

 from the distended epidermis of the under surface of the leaf. Thorough investigations 

 were not made \ 



IXTERCFXLULAR SPACES CONTAINING AIR AND WATER. 



Sect. 51. Air- and water-containing intercellular spaces occur, on the one hand, 

 in many vascular bundles, and these will be treated of in Chap. VIII; on the other 

 hand, they are a characteristic component of large masses of thin-walled assimilating 

 Parenchyma. Intercellular spaces are absent only when the parenchyma forms 

 definite sheaths. 



The cavities in question extend between all cells, so that each one of the latter 

 borders on one or several. They together form, as will be again mentioned below, a 

 continuous system throughout the plant, which opens into the stomata, where these 

 are present. The spaces sometimes contain water in the vicinity of the water-pores, 

 elsewhere they normally contain air, i. e. a mixture of gases similar to atmospheric 

 air, in which the proportion of oxygen and carbonic acid varies with the activity of 

 the processes of assimilation and respiration ^ 



The whole volume of the air-spaces varies greatly in special cases, and is often 

 very large in proportion to the volume of the part of the plant which is not filled with 

 air. Approximate measurements, which Unger^ made on leaves and petioles of 

 41 species of plants, gave as minimum 77 parts by volume of air to 1000 parts of 

 the leaf in Camphora officinalis, and as maximum 713 to 1000 in Pistia texensis. 

 The air of the vessels and that diffused in the cell-sap, which would be also pumped 

 out, is not taken into account in these statements : but it appears that this is on the 

 whole of minor importance, and that on the other hand the figures obtained would be 

 sometimes lower, sometimes much higher, if the single masses of parenchyma com- 

 posing the leaf were investigated separately. 



It was long ago known that the whole volume of the air-spaces in relation to 

 that of the whole plant is largest in plants of all classes and families which grow in 

 water, or in moist positions, and on the other hand in those which inhabit dry places, 

 as many Compositse, Umbelliferae, Labiatse, Grasses, &c. with hollow stems or 

 petioles. 



According to their gradual, and not distinctly limited differences of relative 



* [See further, Russow, iiber sekretfiihrende Intercellular-gange der Acanthaceen, &c., Dorpat . 

 1880.— Bot. Centralbl. 1881, Bd. 5. p. 365.] 



* Compare Sachs, Experimentalphysiologie, p. 262. 



* Beitr. z. Physiol, d. Pflaiizen. I. Sitzgsbr. d. Wiener Acad. Bd. XII. p. 367. 



