DISEASES OF PLANTS 87 



cells undergoing enlargement while the intercellular spaces 

 appear to become filled with mucilage, giving the tissues first a 

 water-soaked appearance, followed by a gradual darkening to a 

 deep brown colour. The brown patches on the head appear to 

 arise in the same way. Very characteristic is the breakdown of 

 the pith to form an elongated cavity (Fig. 11). 



In field experiments it was found that browning of cauli- 

 flower was completely eliminated by a dressing of 10 lb. of 

 borax per acre. 



Cracked Stem of Celery. A disease of celery, known as 

 cracked stem, is widely distributed in the United States and 

 Canada. It was first recorded in Florida and described by 

 Purvis and Ruprecht (1935, 1937). The first external symptoms 

 of the disease are a brownish mottling of the leaf, first in the 

 marginal region, and brittleness of the stem. Next, cracks 

 develop transversely across the leaf stalks which then curl 

 outwards and turn brown. The roots suffer the same discolora- 

 tion and finally die. In extreme cases the terminal bud may 

 suffer the same fate. Where the disease is prevalent it may be 

 very serious, and bring about a loss of half the crop in bad cases. 

 Investigations by Purvis and Ruprecht, including the use of 

 water cultures and sand cultures, as well as field trials, indicate 

 that cracked stem results from boron deficiency and can be 

 controlled in the field by the application of borax either as a 

 soil dressing or as a spray. 



As far as I am aware cracked stem of celery has not been 

 recorded in Britain, although I have seen examples of it induced 

 artificially by Mr Morley Davies through heavy liming of the soil. 



Lucerne Yellows or Yellow Top. The effects of boron 

 deficiency on lucerne (alfalfa) were described by McLarty, 

 Wilcox and Woodbridge (1937) as- a uniform yellowing of the 

 terminal leaves, or a bronzing of the intervenal areas, poor 

 development of internodes and death of the growing points. It 

 appears that the symptoms may be confused with those resulting 

 from attacks by the potato leaf hopper (Empoasca fabae), and 

 recently Colwell and Lincoln (1942) have published a compara- 

 tive account of the symptoms produced in lucerne by boron 

 deficiency and by the leafhopper, their account being the result 

 of work carried out both in the greenhouse and under field 



