MOVEMENTS OF GROWTH. 



377 



of Phaseolus multiflorus which is just beginning to form the first 

 secondary roots. We perceive the epidermis, the cortex, and 

 the central cylinder enclosing the vascular bundles. This is sur- 

 rounded in its entire circumference by a characteristic tissue 

 which we term the endodermis. It is specially characteristic of 

 roots that the wood and bast of their vascular bundles are ar- 

 ranged in a manner essentially different from that in shoots. 

 There are, viz., in roots several xylem bundles, with which al- 

 ternate as many phloem bundles situated nearer to the periphery 

 of the central cylinder. A transverse section of a bean root shows 

 us four xylem and four phloem bundles. We consequently term 

 it a tetrarch root. As with advancing development the root 

 increases in thickness, the tissue between the xylem and phloem 

 bundles is converted into cambium. A closed cambium ring 

 originates, which yields on the inside secondary wood, on the 

 outside secondary bast 



153. Apparatus for Measuring Growth Movements. 



For demonstrat- 

 ing growth move- 

 ments, an arrange- 

 ment first employed 

 by Sachs, and 

 known as the arc 

 indicator, is very 

 serviceable. This 

 piece of apparatus, 

 represented in Fig. 

 123, in the form 

 constructed by 

 Pfeffer, may be ob- 

 tained of Albrecht 

 in Tubingen at a 

 price of 60 mks. 



The thread, /, 

 connected with the 

 plant, is carried 

 over the pulley, r, 

 which is fixed at 



the centre of 

 quadrant, q. 



the 

 I 



FIG. 123 Arc indicator. (\fterPfefler.) 



