INTERCELLULAR SECRETORY RESERVOIRS. 205 



layers, the process of breaking down and of coalescence progresses centrifugally 

 (comp. Fig. 86, and above, p. 69, Fig. 22). 



This mode of development holds for the gum-passages in the periphery of the 

 petiole of the Marattiaceae, for the mucilage-passages of the Opuntias, which require 

 further investigation, and perhaps also of the IMamillarias ; again for all secretory 

 cavities investigated, with the exception of those of the Lysimachias, Myrsinese, and 

 species of Oxalis. Doubtful cases will be named below. 



The schizogeneiic spaces (comp. Fig. 85, p. 203) appear sometimes between 

 cells, which resemble those surrounding cells which do not border on secretory 

 reservoirs, both in arrangement and origin : sometimes they are produced by peculiar 

 divisions of special initial meristem-cells. The first case is found to occur in the 

 above-mentioned spaces of Lysimachia Ephemerum, and in the slit-like transverse 

 passages of Cussonia also noted above. Further, the large longitudinal passages in 

 the secondary cortex of the same plant and of other ligneous plants arise between the 

 common corners of junction of four rows of cells, which are produced from the 

 cambium in the same way (Chap. XIV) as its other products. The same holds, with 

 many special modifications it is true, for the formation of the resin-passages in the 

 secondary wood of the Abietineae ^. The prismatic longitudinal passages at the inner 

 limit of the primary cortex in the roots of Compositae are, excepting in the appear- 

 ance of the characteristic contents, formed in the simplest and most common case, in 

 the same way as those air-containing cavities found in the external layers of the 

 cortical parenchyma, viz. at the corners of contact of four rows of cells. 



On the other hand, the primary passages in the pericambium of the roots of the 

 Umbelliferae, which will be described later (Chap. XIII), are the result of special meri- 

 stematic divisions. The passages in the primary cortex of the Abietineae, and in the 

 leaves of Cycas and Alisma, may each be traced back, according to Frank and N. 

 IMiiller, to a row of initial cells, which divide longitudinally by successive crossed walls : 

 the four daughter cells then separate at the angle of contact to form the passage. 



As the plant grows there occurs, as has been shortly noted above, a widening of 

 the passage, and growth of the cells which enclose it, in a direction tangential to its 

 surface : this is accompanied by increase in number of these cells by radial division, 

 e. g. from the four original cells to 6 or 8 in Pinus and Alisma (Frank) ; in cases of long- 

 continued growth in thickness of the part, and correspondingly great widening of the 

 passages, the number may reach much higher figures, e. g. cortex of Coniferse, Rhus, 

 Pittosporum, &c. On the other hand the cells surrounding the space may also divide 

 in a tangential direction, and the originally single limiting or epithelial layer may be 

 doubled, or divided into several layers, e.g. in the passages of species of Philodendron, 

 the cortex of Pittosporum, Hedera (Fig. 85), and the leaves of Pinus. This mode of 

 origin of several layers of epithelium from the originally simple limiting layer is not 

 proved for all cases where they occur, and another origin is in many cases quite 

 possible for the outer layers. 



The origin of the secretion contained in the schizogenetic spaces, looking at it 

 from the purely histological side, and neglecting the chemical questions, is in my 

 opinion not clear, and requires further investigation. It is obvious that it, or at least 



1 Sanio, in Pringsheim's Jahrb. IX. p. 99. 



