The Plant Cell 23 



The successive layers in the formation of the cell-wall 

 may be interrupted at certain points or along certain lines, 

 and there will thus result pores or pits of various types. 

 Again, the thickening may be confined to particular re- 

 gions, so that the peculiarities of the wall may be consid- 

 ered due to the deposition of the layers in very limited 

 areas, as in the annular or spiral vessels. In the tracheal 

 tissue these pores may occur in adjacent cells opposite to 

 one another, so that the cells at these points are in reality 

 separated merely by the primary layer. Such connections 

 are important in the transport of water and substances in 

 solution; but it is not the province of the present brief 

 description to make an examination of wall structure, 

 the aim being merely to indicate the mechanism of special 

 physiological interest. 



In the case of the soft-rot of cabbage and other vege- 

 tables, the causal bacillus attacks and decomposes the 

 middle lamellae, so that the organization of the tissues is 

 promptly broken down. Gelatinization also of the cell- 

 wall may occur in seed-coats. Flax, mistletoe, and some 

 other plants exhibit this phenomenon when placed under 

 conditions favorable for germination. 



16. Cell-sap.- -The protoplasm is infiltrated with water, 

 and there are closely associated with it nutrient and other 

 substances in solution. Moreover, as already indicated, 

 there are generally present within the protoplasm some 

 definite " vacuoles," also containing substances in solu- 

 tion ; and such solutions are called cell-sap. These vacu- 

 oles are apparently of much the same nature as the large 

 central one ultimately formed in the great majority of 

 differentiated cells. The term "cell-sap," at any rate, is 



