542 lewis. 



cells cut in serial sections, provided the cells are large enough and 

 their Avails distinct. Of human tissues, the epithelium lining the oral 

 cavity is quite favorable, and reconstructions of eight cells have been 

 completed; but the cells are small as compared with those of amphib- 

 ians, and both small and irregular in contrast with cells of vegetable 

 parenchyma. Hence the pith of the elder, Sambucus canadensis, was 

 chosen. If a fresh moist specimen is obtained, it can be imbedded 

 thoroughly in paraffin and cut in perfect series at a thickness of 

 ten microns, thinner sections being undesirable. The chief difficulty 

 will be in interpreting the cell walls which fall nearly in the 

 plane of section and consequently appear as hazy films, but in ten- 

 micron sections these will be dense enough to be recognized readily. 

 Portions of pith were cut transversely and longitudinally, but the 

 reconstructions were made exclusively from longitudinal sections. 

 On the average a cell extended through sixteen of these sections. A 

 small area of the tissue was drawn with the projection lantern at a 

 magnification of 250 diameters, and selected cells in the drawing were 

 enlarged by the pantograph to 500 diameters. After all the contacts 

 had been studied and recorded, the models were made in wax by 

 Ethel S. Lewis, — a task requiring patience and skill beyond that at 

 the author's disposal. The most instructive of these models have been 

 beautifully and accurately drawn by F. Schuyler Mathews. 



The appearance of the cells in elder pith when sectioned lengthwise 

 of the stem is shown in Figure 2. The long axis of the cells is trans- 

 verse to that of the stem, contrary to the early statement of Hooke, 

 but in accordance with a drawing by Dippel, who shows, however, 

 that in the youngest internode in autumn the long axis may be per- 

 pendicular. Possibly after a period of rapid growth chiefly in length, 

 the cells continue to divide transversely, thus becoming compressed 

 and flattened, as found in the older internodes. The cells then have 

 an accordion-like arrangement which is demonstrated by children in 

 removing the pith by pushing it out of the stem. A long stretch may 

 thus be cramped into a short space, but it regains almost fully its 

 original length when released from the stem. It may well be that a 

 similar but moderate contraction in the paraffin has rendered the cells 

 modeled somewhat flatter than is normal, but this would not alter the 

 number or shape of their contacts. The intercellular spaces, essential 

 for the life of the cells, serve to blunt the edges and angles of the 

 polyhedra, the form of which it is our purpose to make clear. Conse- 

 quently these relatively small spaces (shown in Figure 2) have been 

 disregarded. Further, the waviness of some walls, when evidently 



