96 HISTOLOGY OF ANGIOSPBRM STEM, ROOT, ETC. 



zone of colleiicliyma, then thin-walled cortex paren- 

 chyma in which there are resin-ducts, each surrounded 

 by a small-celled epithelium (resin-secreting) layer; 

 (4) the sclerenchyma forms separate strands, one imme- 

 diately outside the phloem of the larger primary vascular 

 bundles ; (5) just outside the sclerenchyma, and traceable 

 as a wavy band round the whole stem, is the endodermis, 

 a layer of cells with wavy cutinised walls and containing 

 starch grains ; (6) chloroplasts occur in the cortical cells 

 and also in the epidermis ; (7) each bundle consists of 

 phloem externally, xylem internally, and cambium in the 

 middle there is no inner phloem. 



105. Interfascicular Cambium in Sunflower. Cut, 

 examine, and compare transverse sections taken through 

 different parts of the stem of a Sunflower plant, or from 

 plants of different ages. Note that in all except quite 

 young stems, or the youngest parts of a fully- grown stem, 

 the individual bundles are more or less joined up by inter- 

 fascicular cambium, which has been formed by the growth 

 and division of the parenchyma between the primary 

 bundles i.e. the parenchyma of the primary medullary 

 rays and is continuous with the primary or fascicular 

 cambium lying between the xylem and phloem of the 

 bundles themselves. In older stems, therefore, there is a 

 continuous cylinder of cambium round the stem, and this 

 gives rise to secondary xylem internally and secondary 

 phloem externally. 



106. Study of Stem of Aristolochia. One of the 



very best Dicotyledonous stems for detailed study is that 

 of Aristolochia (Dutchman's Pipe). A well-grown plant 

 should be obtained from a nursery and cultivated either in 

 a greenhouse or in a sunny position in the garden, pro- 

 vided with trellis or other supports ; or material may be 

 obtained from dealers in botanical supplies. 



The following preparations should be made : (1) A 

 series of transverse sections to illustrate the progressive 

 development of the tissues from the primordial meristem 

 at the apex downwards. Near the apex cut the sections 



