86o A TEXTBOOK OF THEORETICAL BOTANY 



The effect of a common influence extending from cell to cell is also seen 

 in the formation of Sanio's trabeculae. These are bars running horizontally 

 across the lumen of tracheid cells. They are always radial and may run 

 continuously across several cells, even across the cambium into the sieve 

 tubes of the phloem. They originate as folds in the radial walls of the cambial 

 cells, followed by a partial solution of wall substance which separates them 

 from the wall, while in the xylem cells they become lignified. If they serve 

 any specific purpose it is probably that of support against radial growth 

 pressures. They are common in Conifers, but have also been found in a few 



Dicotyledons. 



The transverse expansion of metaxylem elements is associated with 

 the cessation of elongation in the internode, but this is not a sudden change, 

 so that gradations between protoxylem and metaxylem naturally occur. 

 Typical of the transitional zone are the reticulate elements, in which the 

 spiral band, characteristic of protoxylem, gives place to a network of lignin, 

 derived apparently from the close turns of an unstretched spiral, which have 

 become connected at intervals, enclosing areas of unlignified wall. 



The first protophloem elements appear in connection with the young 

 leaves between the third and sixth from the apex. They are true sieve tubes, 

 but small and without companion cells, and they are usually obliterated 

 at about the level of the twelfth leaf. Like the protoxylem they are thus 

 characteristic of the zone of elongation. They are replaced by the larger 

 and longer-lived metaphloem sieve tubes, which have companion cells. 



Sieve tubes form anastomosing strands of elongated, thin-walled cells, 

 their essential distinction being that their contents are connected longi- 

 tudinally through the perforations of the sieve plates. Protoplasmic con- 

 nections between sieve tubes and companion cells or parenchyma cells may 

 also exist, but they are of the common type and have not usually the structure 

 of true sieve plates. The separation of the companion cell from the sieve 

 tube mother cell is one of the last steps in phloem differentiation, and the 

 separating wall may thus remain very thin and is sometimes without pits or 

 plasmodesma. 



The development of a sieve tube cell is marked by the disappearance of 

 the nucleus and by a partial breakdown of the cytoplasm, which becomes 

 completely permeable, coarse and ill-defined. Although still living in the 

 mature element and capable of cyclosis, it is moribund and does not survive 

 for long. The products of disintegration accumulate as slime in the central 

 vacuole and give rise to the " slime plugs," which may be seen in fixed material. 



The ontogeny of the sieve plates cannot be described with certainty, but 

 the mature plates have some recognizable features in common. There is a 

 basic framework of cellulose, in which are large perforations, so that it forms 

 a network. The meshes are lined with " collars " of a polysaccharide of 

 uncertain composition called callose (or callus). Through the tubes thus 

 formed the protoplasm of the adjacent elements is continuous. The amount 

 of callose tends to increase, at first lengthening the connecting pores through 

 the plate, and then narrowing and at last closing them. A massive cushion 



