28 FORMS OF PLANT-CELLS 



(p. 36). In the mature condition they appear as long wide 

 tubes or cylinders, but they are actually derived from vertical 

 rows of cells by the more or less complete breaking down of the 

 cross-walls. Perforated septa thus occur at intervals in the 

 course of the vessel, and in woody plants are often obliquely 

 radial with reference to the organ as a whole. The cross-walls 

 may almost entirely disappear (as in the Maple and Oak, 

 Fig. 17, F), but most commonly a distinct rim persists, only the 

 central part being absorbed (e.g. the Lime or the Poplar, 

 Fig. 17, E). In still other cases the walls break down in such 

 a way that a number of cross-bars remain (e.g. the Alder, 

 Fig. 17, D). 



The elements of the fourth type likewise serve for conduction, 

 in this case of elaborated food-materials. They are known as 

 sieve-tubes, 1 and are located in the bast or phloem, a tissue that 

 in most stems lies just outside the wood. The sieve-tubes, like the 

 vessels, are derived from vertical rows of cells whose cross-walls 

 are perforated by a large number of fine pores through which the 

 protoplasts of the adjacent units become connected. They retain 

 their living contents in the mature condition, and the membranes 

 remain relatively thin (Fig. 13). When the cross-walls are seen 

 from the surface they present the appearance of a sieve, the 

 meshes of which constitute the pores ; these walls are spoken of 

 as sieve-plates (Fig. 13, s.p.). 



Before the sieve-plates are fully developed each cell undergoes 

 longitudinal division into two unequal portions. Of these the 

 larger constitutes a segment of the sieve-tube whose nucleus 

 degenerates, whilst the smaller, which retains its nucleus, forms 

 a so-called companion cell (Fig. 13, c.c.). In some cases two, or 

 even three, companion cells may be produced before the nucleus' 

 of the sieve-tube segment dies away. The narrow companion 

 cells have dense granular contents in contrast to the wider 

 sieve-tubes, which possess but a thin lining layer of cytoplasm 

 and a large central vacuole, features by which they are often 

 readily recognised in transverse sections. 



Behind the growing point the cells have been seen to exhibit 

 a steady increase in size until they attain their full dimensions. 



1 See also p. 82. Sieve-tubes of a different type occur in Ferns and 

 Gymnosperms (pp. 293, 339), 



