6 FORM AND AEEANGEMENT OF CELLS. 



In general, no visible openings can be detected in cells, although 

 fluids pass readily into and out of them. Harting and Mulder, how- 

 ever, state, that they have observed perforations in the cells of Hoya 

 carnosa, Asclepias syriaca, Oycas revoluta, Virginian spiderwort, and 

 Traveller's joy. In one cell (from a Euphorbia), having a transverse 

 diameter of 0-03777 millimetres,* they counted 45 minute holes. In 

 some mosses, also, openings have been found in the cells, as in 

 Sphagnum and Leucobryum glaucum. 



PoEOUS OR Pitted Cells are those in which the membrane is 

 thickenedatcertainparts,leavingthinrounded spots intervening, which, 

 when viewed by transmitted light, appear like perforations or pores 

 (figs. 21, 28). The unequal deposit of the internal en- 

 crusting cellulose or woody matter, is the cause of this 

 condition. The pores of contiguous " cells usually corre- 

 spond as regards position, and sometimes the membrane 

 becomes absorbed between them, so as to allow a direct 

 communication by means of lateral canals, as is seen in 

 the cells from the root of the Date (fig. 20, aa). When 

 porous cells are united end to end, so as to form tubes, 

 the tissue is denominated articulated Bothrenchyma or 

 Taphrmchyma (jSo'^jos and rapjos, a pit), on account of their bead- 

 like appearance, and the pits or depressions in their thickened walls 

 (fig. 22). Pitted cells are seen in Elder pith. 



PiBEOiTS OR Spiral cells are those in which there is a spiral 

 elastic fibre coiled up in the inside of the membrane (fig.' 23). When 

 united they toim fibro-cellular tissue, or Inenchyma (ivig, fibres). These 



Fig. 23. 



Kg. 24. 



Fig. 25. 



Fig. 26. 



cells generally consist of membrane and fibre combined, but the 

 former appears to be sometimes absorbed wholly or partially during 

 the progress of growth. The menibrane, in some instances, is easily 

 dissolved by water, and then the elastic close convolutions of the fibre 

 spring out with considerable force, as in the outer covering of the seeds 

 of Collomia linearis, and in the pericarp of Salvia. Spiral cells 



Fig. 21. Porous cell, from the Elder (Samhucus nigra). Fig. 22. ArtictQated Both- 



renchyma, or Taphrenchyma, from Mistleto, having a moniliform appearance. Figs. 23, 

 24, 25. Spiral, annular, and reticulated ceUs, from Mistleto (Vis(mm albmn). Fig. 26. 



Scalariform and dotted cell, from Elder {Sambuetts nigra). 



■ A millimetre is about l-25th of an English inch. 



