INTUMESCENCES AND WARTS 41 



INTUMESCENCES AND WARTS 



These structures frequently occur on the surface of leaves 

 or on young shoots, and may be due to the work of mites or 

 insects ; on the other hand they may appear as the result of 

 special conditions of temperature, moisture, etc., or again 

 they may be directly produced by the application of some 

 chemical stimulant, as in spraying. Those cases resulting 

 from the influence of some physical agency will only be dealt 

 with here. 



In this country such small warts are most frequently met 

 with on the under surface of vine leaves grown under glass. 

 It is generally agreed that excess of temperature and humidity 

 favour the formation of these growths. Opinions differ as to 

 the part played by light in the process. Sorauer and Atkinson 

 consider that a dull light favours the formation of intumes- 

 cences, whereas Dale states that white light, more especially 

 yellow and red rays, are absolutely necessary for their forma- 

 tion. Viala and Pacottet also state, as the result of experi- 

 ments, that intumescences are caused by an excess of light in 

 a damp atmosphere, and that it is only during a period of 

 bright light, and only on leaves near to the glass that these 

 structures are formed in quantity, being practically absent 

 from leaves growing in diffused light or in the shade. These 

 authors consider that shading the glass would prevent the 

 formation of warts by checking excessive transpiration. 

 Atkinson, on the other hand, considers that their presence 

 may be due to the greater turgescence of the leaves owing to 

 the low rate of transpiration in a gloomy greenhouse. It is 

 evident from the foregoing statements that the part played 

 by light in the formation of warts is by no means settled. Its 

 direct action, if of any importance at all, may vary with 

 different plants. 



Sap warting. This is a term used by gardeners to express 

 the appearance of small ruptures and corky outgrowths in the 

 bark of the stem or branches, which are sometimes continued 

 along the midrib and veins of leaves. At first the ruptures 

 are scattered and might be mistaken for lenticels, but usually 

 they continue to increase in number until the branch is 

 almost covered with small, gaping cracks with raised edges, 

 and showing the inner pale cells of the bark in the central 



