56 MORPHOLOGY OF THE ANGIOSPERMS 



conducting cell, the sieve cell, is elongate, and has walls with minute 

 perforations, through which extend, in restricted areas, cytoplasmic 

 strands connecting the protoplast with the protoplasts of adjacent simi- 

 lar cells. With specialization, the sieve cell structurally parallels the 

 vessel element; it becomes progressively shorter, with end walls more 

 and more oblique, until they stand finally at right angles to the side 

 walls. Where sharper delimitation sets apart a group of sieve areas, 

 sieve plates are formed. These plates indicate lines or courses of con- 

 duction in the sieve cell, a first step in the formation of a linear group 

 of these cells, a sieve tube. ( Sieve cells that have sieve plates and unite 

 to form a sieve tube are termed sieve-tube elements.) With increased 

 specialization, the sieve plates become more and more prominent, their 

 pores larger and fewer, and the plates become restricted to the end 

 walls of the cells. Where the end wall is transverse, the plate occupies 

 most of the wall and the pores are few and large. A cylindrical cell of 

 this type is the highest form of sieve-tube element, resembling in form 

 the most highly specialized vessel element. With the elaboration of 

 sieve plates, the other sieve areas remain weakly developed or vestigial 

 — "ghost plates" or lattices. Advanced woody families, herbs, vines, and 

 plants of unusual habit usually have sieve-tube elements of the highest 

 type. 



The companion cell, a new cell type in the phloem of vascular plants, 

 is characteristic of angiosperms. No companion cells have been found in 

 Austrobaileija (Fig. 26). Companion cells are parenchyma cells of spe- 

 cial structure and function, closely related in position and function to a 

 sieve element. They lie alongside the sieve elements and are pitted only 

 with those cells — evidence that their function is closely related to that 

 of the sieve elements. Companion cells are characteristic of angio- 

 sperms but absent in other seed plants. 



Usually one or more companion cells accompany each sieve element; 

 rarely, a sieve-tube element has no companion cells. The number of 

 companion cells appears to increase with increase in specialization of 

 the phloem, and the phloem of herbs and many woody taxa may have 

 numerous companion cells. In general, highly specialized phloem ac- 

 companies highly specialized xylem. The correlation in advancement 

 between xylem and phloem is often closer than that between flower and 

 vascular tissues. 



Phloem is usually a complex tissue, including phloem parenchyma 

 and sclerenchyma of one or more types, in addition to sieve tubes and 

 companion cells. In the more primitive woody angiosperms, the phloem 

 tends to be simpler than in the more advanced families and has fewer 

 companion cells and much less sclerenchyma. The phloem of some of 

 the woody Ranales is "soft," not "stratified" with alternate bands of 



