48 Thompson — The Stnicturc and Dn>elopment of 



They bend outward along a radius of the bundle, and in a 

 definite position, at about one half of the distance from the 

 periphery to the mid-line of the bundle. After the passage of 

 these strands, there is no further trace of internal phloem in 

 the petiole or leaf. 



Histology of the Root. 



The structure of a very young root, in transverse section, is 

 illustrated in Plate IX, Fig. 3. The loose-celled starch-bear- 

 ing cortex, about seven to eight cells deep, is separated by a 

 thin-walled endodermis from the axial vascular cylinder. The 

 bundle is typically diarch. The two groups of the protoxylem 

 consist each of about six spiral tracheae, and between them 

 at the sides of the bundle lie two small patches of phloem, 

 separated from the protoxylem by the procambium, a layer 

 of large prominently nucleated cells. Outside the xylem and 

 phloem elements, and just within the endodermis, is the 

 pericambial zone. Later, by secondary growth, the xylem is 

 united into a central cylinder, surrounded externally by a ring 

 of phloem, but internal phloem is entirely absent in the root. 



On older roots irregular warts or swellings are frequently 

 found, which, when sectioned, reveal a vigorous fungoid growth. 

 The fungoid hyphae ramify through the cells of the inner and 

 especially the middle cortex, and in some places large cavities 

 occur, resulting from the breaking down of the cortex cells. 

 These are filled with the coiled hyphae and the fructifications 

 of the fungus. Starch is usually absent in the cells inhabited 

 by the fungus. In the root of a seedling about six weeks old, 

 the fungus was already well established in many cortex cells. 



Histology of the Seedling. 

 The diarch condition of the root is continued in the hypocotyl, 

 and it may at once be stated that the median plane of the two 

 protoxylem masses corresponds to the median plane of the 

 cotyledons. The spiral tracheae of each end have at first a 

 Y-shaped arrangement, the arms of the Y pointing toward 



