FORM AND ARRANGEMENT OF CELLS. 



are occasionally absorbed so as to form continuous tubes. When 

 cells are united in a rectilinear manner, those in contiguous rows are 

 either directly opposite to each other, that is, are placed at the same 

 height (fig. 18), or are alternate, from being placed at different heights 

 (fig. 19); cells sometimes communicate with each other laterally (fig. 

 20 a a). Isolated cells, as spores of sea-weeds, occasionally have free 

 filaments, or cilia (cilium, an eyelash) developed on their surface. 



18 



19 



8. The simplest kinds of plants, as mushrooms and sea-weeds, are 

 composed entirely of cellular tissue, and are called Cellulares. The 

 pulpy and succulent parts of all plants contain much cellular tissue, 

 and the object of horticultural operations is to increase the quantity of 

 this tissue in ordinary fruits and vegetables. The pith of trees and 

 plants during their early development is cellular ; so also are cotton 

 and rice-paper. 



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

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

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

 Asclepias syriaca, Cycas 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. 



10. Porous ceils are those in which the membrane has been thick- 



ened at certain parts, leaving thin rounded spots, which, 

 when viewed by transmitted light, appear like perfora- 

 tions or pores (figs. 21, 28). The pores of contiguous 

 cells usually correspond as regards position, and some- 

 times 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 



Figs. 17, 18, 19. Cells united together by their extremities. 



Fig. 20. Elongated thickened cells, from the root of the Date palm, o a, Canals of communi- 

 cation. 



Fig. 21. Porous cell, from the Elder (Sambucus nigra). 



Fig. 22. Articulated Bothrenchyma, or Taphrenchyma, from Misletoe, having a inoniliform 

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



