120 THE STEM 



Notice that parenchyma cells of the cortex have intruded into the 

 gaps between the strips of collenchyma cells. 



(r) The pnvionslij continuous cylinder of ijcricjjclir filxrs 

 has likewise been ruptured and the gaps filled up by the paren- 

 chyma cells from the cortex. Notice the beginnings of wall- 

 thickness and lignification in some of the parenchyma cells be- 

 tween the strips of fibers; these parenchyma cells will finally 

 become thick-walled sfone-ceUs (by a process of secondary 

 sclerosis) and will thus effectively "repair" the broken me- 

 chanical cylinder. (For further details of this phenomenon, cf. 

 the Introduction of Exercise VIII.) 



(d) The medullary rai/s between the vascular bundles have 

 become broad and long and their component par(Mi('hynia cells 

 are arranged in more or less definite radial raws as the result of 

 the continued activity of the intrrfascicular canihiuin. 



(e) Each va.scular bundle has increased enormously in size 

 as the result of the continued activity of the fascicular cambium. 

 Notice the crushed condition of the cells in the outer portion of the 

 phloem: this has resulted from the centrifugal development of 

 additional secondary phloem. The xylem portion of each vascular 

 bundle shows three definite annual rings; notice that the largest 

 vessels occur at the edge (i.e., in the "spring wood") of each 

 annual ring. 



(/) The irregularly-shaped 29?7/( is now much reduced in ex- 

 tent and shows clear indications of crushing. Aristolochia repre- 

 sents one of the exceptional cases where the jiitli is actually com- 

 pressed as the result of secondai-y growth. 



III. Suggested Drawings and Notes. — 



1. Pre])are a large diagrammatic drawing of the trans-section 

 of the stem of Pelargo)iiu)n indicating by legends or by labels 

 the position and relative extent of all th(> tissues and regions. 



2. Prepare a drawing similar t(^ 1lie jibovo of the trans-section 

 of the stem of Tilia. 



3. Di-aw a lenticel (of Samhucus. or Aristolocliia) as seen in 

 median longi-sectional view showing carefully the complementary 

 tissue and the closing layers and the relation of Ihese tissues to 

 the adjacent periderm of the stem. Label all important struc- 

 tures. 



