GENERAL CONSIDERATIONS. 11 



seml)Uince to cells, but in other cases cells which seem to belonj^ to 

 the parenchviiia are tilled relatively early without beiujj;- crushed. 

 These are generally nonlionitied wood cells bordering on or lying in 

 the vicinity of the vascular t)UTidles. Probably the organisms l)ore 

 their way through the cell wall in the manner described by Potter, but 

 the writer has never been able to make out clearly any such penetration. 



The writer will be pleased at any time to show the slides from wiiich 

 these photographs were made to anyone who is interested, and in 

 exceptional cases will mail slides, or material from which sections may 

 be cut, to those who are particularly interested, this especially because 

 all reproductive processes are imperfect, the gelatin prints being 

 inferior to the negatives and the latter to the microscopic image of 

 the sections. Most of the sections were cut and the photographs made 

 early in 1901, but other work got in the way and delayed the comple- 

 tion of the paper. Most of the negatives were, however, exhibited 

 in the form of lantern slides at several ])laces early in 1901. e. g.. 

 University of Michigan, Michigan Agricultural College, The Botanical 

 Seminar of Washington, and the sul)stance of the paper was presented 

 before the Society for Plant Morphology and Physiology at its tifth 

 annual meeting in 'New York, December 30, 1901, where the lantern 

 slides were again exhibited. 



The following statements will be made plainer by a brief account of 

 the structure of the turnip root. As may be seen from Pis. II and 

 111, the basal and swollen part of the turnip root consists of a small 

 pith, a large cylinder of xylem, a narrow cambium cylinder, and a 

 phloem cylinder, beyond which is a cylinder of cortical parenchyma 

 surrounded by cork. In other words, the structure is that of many 

 dicotyledonous stems. The upper part of the root has a larger pith 

 and better differentiated medullary rays. The xylem part of the root 

 contains much wood parenchyma, which is not always easily distin- 

 guishable from the medullary rays. The only lignified parts are the 

 reticulations in the vessels. These stain a bright pink with safranin 

 and come out quite distinct from the surrounding wood parenchyma 

 in man}" of the photographs. 



PLANT FURNISHING THE CULTURES. 



On PI. I, fig. 1, is shown the cross section of a turnip root, the 

 interior of which has been destroyed by the bacterium of the brown 

 rot, PKeudomonas campestris (Pammel) Smith. This root was collected 

 in September, 1896, from a field near Baltimore. 



Turnips attacked by brown rot often live for a long time, but the 

 diseased root does not enlarge much radially. This one, like many 

 others observed by the writer, had the form of a carrot root rather than 

 that of a turnip root, although it was several months old. The walls of 

 the cavities in such roots are usually black or brown, and hence the 



