3i8 THE STRUCTURE OF ECONOMIC PLANTS 



in which the cells are much elongated tangentially, and the 

 external layers of the pericycle function as an active phellogen 

 producing a well-defined periderm of regularly arranged and 

 flattened cells. Scattered lenticels may occur over the surface of 

 the root. 



In connection with a study of the plugging of alfalfa roots 

 caused by wilt or root rot, LeClerg and Durrell (17) have described 

 the tissues of the root. In the secondary xylem, the vessel seg- 

 ments are very short and the walls are densely beset with bordered 

 pits. These are transversely elongated, in some instances to such 

 an extent that the wall appears to have scalariform or reticulate 

 thickenings while in others the pits are more nearly circular in 

 outline. The end walls of adjacent vessel segments are perforated 

 with many minute pores. The walls of mature fibers are so thick 

 that the lumen of the cell is nearly occluded. 



Graber and his coworkers (14) have pointed out that, "The 

 alfalfa plant has a high metabolic activity, with great ability 

 for regeneration; and its reserves are rapidly reduced by frequent 

 cuttings." There is a marked difference in alfalfa roots grown 

 under environmental conditions and cultural practices which pro- 

 duce high reserves as compared with those which result in low 

 reserves. In general, roots of high reserve plants have a greater 

 proportion of parenchyma in relation to the fibrous tissue; and 

 the parenchymatous cells are packed with reserve foods, principally 

 starch. 



Root Nodules. — The general considerations regarding the 

 root nodules of the Leguminosae, including method of infection, 

 the penetration of the bacteria into the tissues, and the growth 

 of the nodule, are discussed in the succeeding chapter on Pisum, 

 but certain points specifically dealing with the nodules in Medicago 

 deserve mention. 



Thornton (30) has investigated the early development of the 

 nodule in alfalfa and on the basis of Milovidov's (ii) classifica- 

 tion of types of infection, places Medicago in type I in which 

 "most, if not all the cells of the infected region receive their 

 bacteria by separate intrusions of the infection thread." The 

 infecting organism in this case is Rhizobium meliloti which, 

 according to Fred, Baldwin and McCoy (10), is apparently specific 

 for the alfalfa group, including six species of Medicago, three 

 of Melilotus, and one of Trigonella. The bacteria usually enter 



