HISTOLOGY OF ANGIOSPEBM STEM, ROOT, ETC. 97 



at short intervals in order to trace the appearance of the 

 first xylem and phloem elements to be differentiated from 

 the tissue of the procambial strands. Farther back take 

 sections from successive internodes. (2) A series of longi- 

 tudinal sections through the same regions. 



Treat the sections with aniline sulphate, etc. Note 

 where the first elements of the vascular bundle appear in 

 the procambium ; how the earliest vessels become stretched 

 longitudinally ; how far back from the apex the original 

 cellulose walls become lignified in the xylem and the 

 sclerenchyma ; how the cambium becomes a continuous 

 zone by the development of inter fascicular cambium ; how 

 the interfascicular cambium produces secondary medullary 

 rays, which do not extend inwards to the pith ; how the 

 pith becomes crushed as secondary growth proceeds ; how 

 the originally continuous sclerenchyma band becomes 

 broken up into strips; how the cork cambium arises in 

 the cortex and produces cork ; and so on. 



107. T. S. Stem of Maize (Figs. 27, 28). Cut thin 

 transverse sections of one of the lower internodes. Note 

 that the bundles, though " scattered," are most crowded 

 towards the periphery, and that in each bundle the phloem 

 is external (nearest the periphery of the stem) and the 

 xylem internal. Note (1) the epidermis of small thick- 

 walled cells ; (2) the narrow zone of sclerenchyma below 

 the epidermis ; (3) the ground tissue parenchyma of 

 thin-walled cells, with small intercellular spaces at the 

 corners ; (4) the bundles, each with a more or less com- 

 plete sheath of sclerenchyma. In a single bundle note 

 the (usually four) conspicuous xylem vessels arranged 

 like a V, thus : 



with narrower vessels lying between; the patch of thin- 

 walled phloem lying partly between the two larger xylem 

 vessels. 



With high power note that the epidermis is covered by 

 a distinct cuticle ; the hypodermal sclerenchyma is inter- 

 p. B. 7 



