DISEASES OF PLANTS 81 



Quite a number of investigations have been made on the 

 histology of boron-deficient plants. These include broad bean 

 (Warington, 1926), tomato (Johnston and Dore, 1929; Fisher, 

 1935; Van Schreven, 1935), Citrus (Haas and Klotz, 1931), 

 tobacco (Van Schreven, 1934), sugar beet (Jamalainen, 1935), 

 maize (Eltinge, 1936), potato (Van Schreven, 1939), apple 

 (Mac Arthur, 1940). carrot (Warington, 1940), Brassica spp. 

 (Chandler, 1941), radish (Skok, 1941), squash (Alexander, 1942), 

 sunflower (Lowenhaupt, 1942), garden beet and cabbage 

 (Jolivette and Walker, 1943). 



In Vicia Faba Warington found that the chief internal symp- 

 toms of boron deficiency were frequent hypertrophy of the cells 

 of the cambium and subsequent discoloration and degeneration 

 of the cells ; disintegration of the cells occurred, however, whether 

 there was previous enlargement or not. Disintegration of 

 phloem and ground tissue was also frequent, while xylem de- 

 velopment was poor and the cells might also ultimately dis- 

 integrate. Brenchley and Thornton (1925) had previously found 

 that development of the xylem of the root nodules either failed 

 altogether or was very poor. 



In sugar beet and Citrus hypertrophy of the cambium, fol- 

 lowed by its disintegration and that of the phloem, is also symp- 

 tomatic of boron deficiency. In Citrus boron was found to be 

 essential for meristematic and cambial activity. In tobacco Van 

 Schreven found that the first symptoms appeared in the cells of 

 the root apex and later in the stem apex. In these regions cells 

 became brown and degenerated. The symptoms then extended 

 backwards from the apices, the vascular tissue being par- 

 ticularly affected. Proliferation of the cells of the phloem 

 occurred resulting in the individual elements being compressed 

 and distorted, while the xylem development was poor. Cells of 

 the ground tissue might also undergo disintegration. The same 

 worker recorded similar effects in tomato where thin-walled 

 cells, such as those of cambium, phloem and ground tissue, 

 undergo degeneration. The degeneration of the phloem in 

 tomato was earlier recorded by Johnston and Dore and at about 

 the same time by Fisher. 



A number of species of Brassica were examined by Chandler. 

 In broccoli and Brussels sprouts boron deficiency was found to 



