CORRELATION OF ROOT AND STEM. 227 



a series of very thin sections can, after suitable preparation 

 of the material, be readily taken. 



Comparison of Stem and Root. A comparison of the origin of 

 the tissues in root and stem, as shown in this and the preceding 

 chapter, will enable us to see a resemblance in general plan, com- 

 bined with marked changes in structural detail ; which latter will 

 be more clear by a comparison of the materials of Chapter XII. 

 with those of Chapters VIII. and IX. foregoing. The structural 

 differences, as will be seen, mainly concern themselves with a 

 comparison of the vascular cylinder in the two cases ; and as stem 

 and root are continuous, it follows that at some place or other 

 there must be a transition of the tissues of the one into those of 

 the other. In this transition there is a simultaneous twisting of 

 each xylem strand upon its axis, and a separation of it from its 

 associated bast. The character of the epidermis, also, is changed, 

 the exodermis is differentiated, the endodermis becomes more pro- 

 nounced, and the pith is reduced in quantity. All these changes 

 take place quite gradually, by no . means simultaneously, and to 

 very varying extents in different groups of plants. The relations 

 of the two vascular-bundle systems to one another show that the 

 central cylinder of roots is not a simple vascular bundle, but that 

 a vascular-bundle complex is enclosed in it ; but the comparison 

 further shows that the central cylinder of the root corresponds as 

 a whole with the central cylinder of the stem, a correspondence 

 to which attention was first specifically drawn by Van Tieghem, 

 and which furnished the basis for his " stelar theory," to which 

 attention has more than once been drawn. In both cases the 

 central cylinder is bounded externally by a pericycle, which, how- 

 ever, is, as a rule, much more sharply delimited in roots than in 

 stems, and was thence recognised at a much earlier date under 

 the name of " pericambium ". The tissue of the pericycle is 

 especially capable of new developments, and in it various tissues 

 and new structures have their origin. As in roots the cortex is 

 delimited towards the central cylinder by the endodermis, so in 

 stems it is very commonly cut off by a starch-sheath. In some 

 stem parts this delimiting layer has quite the character of a root- 

 endodermis ; but it may be entirely wanting, so that the limits of 

 the pericycle towards the cortex are no longer capable of ana- 

 tomical definition. 



