MORPHOLOGY OF HIGHER PLAXTS. 



341 



being used as being apparently synonymous with phloem and the 

 latter being equivalent to xylem. As a matter of fact, these terms 

 are not equivalent, the phloem proper including bast fibers in 

 addition to leptome ; and the xylem being composed of wood 

 fibers in addition to hadrome ; nor is the fibrovascular bundle 

 synonymous with mestome strand, as the former includes not only 

 the conducting tissues but the mechanical tissues as comprised in 

 the xylem and phloem ; while the mestome strand includes only 

 the conducting cells comprised in the leptome and in the hadrome, 

 there being no sclerenchymatous fibers present. The following 

 table will doubtless make clear to the student the relationship 

 of these tissues to each other: 



Leptome or 

 Sieve portion 



Mestome or 



vascular 



bundle 



Stereome or 

 Bast fibers 



rSieve tubes / 



< Accompanying cells \ 

 I^Cambiform J 



Cambium 



Phloem 



Fibro- 

 vascular 

 bundle 



Hadrome or 



{Tracheas 

 Tracheids 

 Wood parenchyma 



Libriform or 

 Wood fibers 



Xylem 



When the collateral mestome strand increases in thickness, the 

 increase is due to the activity of the cambium, here called the 

 iNTRAFASCicuLAR CAMBIUM, which then dcvclops phloem or lep- 

 tome outwardly and xylem or hadrome inwardly. Between the 

 primary mestome strands there is frequently a procambium. 

 which connects these strands with each other, and which gener- 

 ally gives rise to secondary mestome strands, or the connection 

 may be efifected by means of the intrafascicular cambium, which 

 often extends itself from one strand to another and develops lep- 

 tome and hadrome, as in the primary strands ; such cambium is 

 distinguished as interfascicular camiuum and is commonly 

 referred to as the camiuum ring. 



The BicoLLATERAL mcstome strands or fibrovascular bundles, 

 characteristic of some Dicotyledons (Labiata\ SolanacecX. Cucurbi- 

 tace^, etc.), differ from the collateral type by having a leptome 



