THE STEM 155 



at the same time curious that the canals of neighbouring 

 cells generally correspond, so that these spots, pores, or 

 pits, as they are called, are simply points where adjoining 

 cells are divided only by the very thin primary membrane 

 through which the sap can easily diffuse. Sometimes 

 even this extremely thin cell-wall disappears, and the 

 cavities of neighbouring cells are brought into open 

 communication with each other. On the other hand, 

 if most of the cell-wall remains unthickened, the thicken- 

 ings appearing on the inner side will present the most 

 varied forms of nets, rings, spirally- twisted bands, etc. 

 (fig. 42, 4 and 7). Such varieties of form in thickenings 

 and pores are generally described and enumerated with 

 special care by micrographers. To physiologists, on 

 the other hand, form as such, however curious it may 

 be in itself, does not present any interest as long as its 

 significance, the part it plays in the life of the plant, is 

 unrecognised ; and this is precisely the case with 

 regard to the different forms of thickening. It is only 

 comparatively recently that attempts have been made 

 to explain these structures from the physiological point 

 of view. We shall briefly return to this matter a little 

 further on. 



These two considerations, viz. the general outline of 

 the cell and the structure of its walls, do not, however, 

 exhaust the ways in which cell structure may vary. 

 Cells can also become fused into more complicated 

 organs, generally known as vessels or tubes. These are 

 generally formed by the perforation or the total dis- 

 appearance by absorption of the transverse partitions 

 between vertical rows of cells. For instance, if a row of 

 cells with spiral thickenings lose the transverse partitions 

 which divide them from each other (fig. 43, i, on the left), 

 a continuous tube called a spiral vessel will appear 

 (fig. 43, i, on right). Sometimes, however, as we have 

 said, vertical rows of cells, in transforming themselves 

 into tubes, do not entirely lose their transverse partitions, 



