58 THE PLANT CELL. 



(C) The Medullary rays (produced in part by certain cells of 

 the cambium). 



In addition, the endodermis, pericycle, and medulla or pith will 

 be briefly described as component tissues of the central cylinder. 

 It should, however, be remembered that in those plants which 

 do not possess a strictly limited central cylinder, or a well-defined 

 zone of cambium, phloem and xylem elements may be met with in 

 the so-called closed vascular bundles, similar in many respects to 

 those occurring in the xylem and the phloem of the more highly 

 differentiated groups — viz., Dicotyledons and Coniferae. 



(A) The Phloem or Bast. 



The cells produced by the cambium on its outer or cortical 

 aspect go to form a tissue consisting almost entirely of elements 

 known as sieve-tubes. The undifferentiated cells originating 

 from the cambium are at first quite thin-walled, but soon changes 

 take place which result in ; — 



(i. ) Thickening of tlie lateral and end-walls. 



(ii.) The formation of special areas known as SieVC-plateS upon the 

 end-walls. 



These sieve-plates are formed aa follows : — thin areas are left in the 

 end- walls during the development of the sieve-tube, and after a time these 

 thin areas, which coincide with one another in adjacent end-walls, unite, 

 the intermediate middle-lamella becoming absorbed. The other portions 

 of the end-walls become much thickened ; and in some cases several such 

 sieve-areas may be present in the end-walls of tubes (Tilia), the number 

 of actual perforations, or pits, which may be present in each sieve-area 

 being perhaps twenty, thirty, or more. 



Sieve-tubes may be readily examined by taking transverse and 

 longitudinal sections of such a stem as that of (JucurUta. In 

 transverse section each sieve-tube is seen to be of a somewhat 

 in-egular shape ; lying just outside the cambial layer, and close 

 to the tube — being, in fact, cut off from the main cell — is 

 to be seen a smaller cell, known as the companion-cell, which 

 appears full of granular contents. In such a section the tubes 

 are usually recognised by their sieve-areas, which may be made 

 more evident by staining the section with eosin (see Fig. 40). 



In longitudinal sections each sieve-tube is seen to be an 

 elongated element, with its narrow companion-cell lying next to 

 it along its whole extent. Towards the middle line of the tube 



are to be seen the contracted protoplasmic contents that is to 



say, if ordinary spirit-preserved material is being used for the 



