376 



PHYSIOLOGY OF GROWTH. 



noticed that the vascular bundles, arranged in a circle, are still 

 separated pretty considerably from one another. The fascicular 

 cambium, /c, of the separate bundles, composed of small radially 

 arranged cells, is continued between them to form the interfasci- 

 cular cambium, ifc, developed from the parenchyma of the ground 

 tissue, and thus a closed cambium ring is produced. 



As the shoot continues to 

 develop, the cambium (here 

 only the fascicular cam- 

 bium) produces on the in- 

 side secondary wood, on 

 the outside bast tissue. 

 The growth in thickness 

 thus brought about results 

 chiefly in a very consider- 

 able increase in the wood of 

 the fibro-vascular bundles, 

 as we at once discern if we 

 examine transverse sections 

 from Aristolochia stems 10 

 mm. in thickness. We also 

 perceive in these perhaps 

 ten primary medullary rays, 

 which traverse the wood in 

 its entire thickness, and 

 therefore extend from the 



cambium to the pith. Secondary medullary rays are present in 

 considerable numbers. It is further to be noted that at the 

 periphery of the older shoots a formation of periderm has set in, 

 and that the closed ring of sclerenchyma present in the younger 

 parts (see Fig. 122, sk) is broken up into separate pieces. 



The hypocotyl of Ricinus communis also affords a favourable 

 object for the study of the changes which take place in stems 

 during growth in thickness. In Fig. 112, p. 309, is depicted the 

 appearance of a transverse section of a vascular bundle of a 

 fully elongated hypocotyl of Ricinus. We use alcohol material, 

 and make out without any trouble the presence of the fascicular 

 and interfascicular cambium. During growth in thickness, second- 

 ary wood is deposited on the inside and secondary bast tissue on 

 the outside. 



We prepare a section from the upper part of a root of a seedling 



FIG. 122. Transverse section of a twig of 

 Aristolochia Sipho, 5 mm. in thickness. TO, pith ; 

 fv, vascular bundles ; vl, their vascular region ; 

 cb, sieve region ; ifc, interfascicular cambium ; p, 

 phloem parenchyma outside the sieve region, 

 affording transition to the ground tissue; pv, 

 pericycle; s/c, ring of sclerenchyma; e, starch 

 sheath; c, green cortex ; cl, collenchyma. Magn. 

 9. (After Strasburger.) 



