2o6 UOTANY AAD i'ilARMACUGNOSY. 



quently with the innermost layer differentiated as an endodermis. 

 The latter surrounds the so-called pericycle, a sheath consisting 

 of more or less distinct stereomatic strands, either forming a 

 closed sheath or merely representing isolated arches outside the 

 leptome of the stele. Inside the pericycle we observe the mestome 

 strands constituting mostly one circular band (in cross section) 

 in the Dicotyledons and Gymnosperms, or several more or less 

 concentric bands in the Monocotyledons. The mestome-strands 

 may be collateral (Fig. 115), bicollateral or concentric, the last 

 of which being found only in the Monocotyledons (Fig. 212) and 

 Ferns (Fig. 278). 



In the DICOTYLEDONS the collateral mestome-strands which are 

 the most frequent, contain leptome, i.e., sieve tubes, companion- 

 cells and cambiform, furthermore cambium, and inside this fol- 

 lows the hadrome, i.e., vessels, tracheids, mestome, parenchyma 

 and libriform. When the collateral mestome-strand increases in 

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

 called the intrafasicular cambium, which then develops lep- 

 tome outwardly and hadrome inwardly. Between the primary 

 mestome strands there is frequently a procambium, which con- 

 nects these strands with each other, and which generally gives 

 rise to secondary mestome strands, or the connection may be 

 effected by means of the intrafasicular cambium, which often 

 extends itself from one strand to another and develops leptome 

 and hadrome, as in the primary strands, such cambium is distin- 

 guished as iNTERFASicuLAR CAMBIUM and is commonly referred 

 to as the cambium ring. 



The BICOLLATERAL mestomc strands, characteristic of some 

 Dicotyledons (Labiatse, Solaneae, Cucurbitacese, etc.) differ from 

 the COLLATERAL type by having a leptome strand developed on the 

 inner face of the hadrome, thus each mestome strand carries two 

 strands of leptome (Figs. 208, 220). In the concentric mes- 

 tome strands, the leptome may encircle the hadrome, as in the 

 Ferns (Fig. 278), or the hadrome may partly (Fig. 212), as in 

 the rhizomes of many Monocotyledons, surround the leptome. 

 While thus the collateral type of strand or bundle occurs in both 

 Monocotyledons (Fig. 114) and Dicotyledons (Figs. 104, 115, 



