4(i THE PLANT CELL. 



cules being larger than those of salts. Later on, during the life 

 of the cell, the walls are generally too thick to allow of the above- 

 mentioned process of absorption, and then the presence of "pits," 

 or perforations in the walls, l>ecomes a factor of great importance 

 in the transfi'rence of food materials and water from cell to cell; 

 and it has already been seen that the cytoplasm also passes 

 slowly from cell to cell by means of the same " pits." 



XoIl — Cells of young developing tissues (meristem) may be studied in the 

 young endosperm of Callha pnliixtrix {see Fig. .5), or in sections of root-tips 

 orapicus of stems The same method of fixing, hardening, and staining may 

 be used as in the preparation of the young undifferentiated cell. (Note 

 at end of Chapter ii.). Callha is the marsh marigold, and the endosperm 

 starts developing from the beginning to the middle of June, after the 

 petals have fallen, and the carpels have just started to ripen. Transverse 

 sections of the tarpels will cut the ovules longitudinally, and a large 

 number of sections may be rapidly examined, the thinnest and best being 

 selected for mounting. 



B. '/J)y\h MEKISTF.MATIG TISSUICS. 



Under this heading are included : — 



i. The Cambium (stem and root). 



ii. Cork-cambium (stems), 

 iii. Perieyele (roots). 



The second of these has been already examined under cork- 

 tissues. The perieyele will be examined in Chap. v. 



In cambium wall-formation dm-ing cell-division takes place in 

 only two directions, generally speaking — viz., the radial and the 

 tangential directions in a stem or root; thus the walls produced 

 in such a tissue have always a fixed orientation, being either 

 situated along a radius, or perpendicular to radii of the organ in 

 which they occur. In rudimentary tissues other than cambium 

 it was pointed out above that the main directions of wall-forma- 

 tion were either synclinal or anticlinal with regard to certain 

 fixed planes in the bud, and that walls might be formed at times 

 in almost any direction in space. In the tissue now to he 

 studied, however, a marked regularity in the directions of wall- 

 formation is preserved. The planes in which walls are formed 

 are always parallel to either a fixed perpendicular, or transverse 

 plane in the organ. 



The statements made with regard to thickening and orowth 



