174 BOTANICAL GAZETTE [august 



gummosis 



is due to a cytase which, unable to attack the wall of a living cell does so as 

 soon as the cell is injured from any cause. He also rejects Ruhland's view 

 that the gum is an oxidation product of carbohydrates and that gummosis is 



of air through wounds. Butler considers that " gum- 



caused by admission of air through wounds, 

 mosis is due to hydrolysis of the walls of the embryonic wood cells, which 

 develop into a susceptible tissue." The form of development of a spot of 

 gummosis shows, however, that it is correlated with release of pressure of the 

 cortical tissues. Gummosis does not occur unless the cambium is growing 

 actively and there is an abundant supply of water available to the roots; when 

 these two conditions are present gummosis may develop " autogenously " or 



stimulates 



Contrary 



lay no part in gum formation. " Gummosis of Prunus and gummosis 

 of Citrus are indistinguishable maladies." Both squamosis and exanthema are 

 considered to be forms of gummosis. An excellent bibliography is appended. — 



Metcalf 



Root habits of desert plants. — In studying the roots of plants growing 

 near the Desert Laboratory, Tucson, Ariz., Cannon 13 has made a rather 

 detailed investigation of more than 60 species, including winter and summer 



types 



type 



agnized 



very 



roots well developed, a specialized type with the tap root the chief feature, 

 and a second specialized type in which the laterals, placed near the surface 

 of the ground, are especially well developed. The cacti are almost the sole 

 representatives of the last type, and represent a specialization of a xerophytic 

 form capable of absorbing a water supply from rains which penetrate a few 

 centimeters only. This type seems necessarily limited to plants with 

 considerable water-storage capacity. A further specialization in the roots of 

 most cacti is to be seen in the development of an anchoring and an absorbing 

 system. 



Plants having prominent tap roots include comparatively few species. 

 They are mostly perennial in habit and limited in their distribution to areas 

 with considerable depth of soil. In contrast, the generalized system is charac- 

 teristic of the majority of both the perennial and annual species. It facilitates 

 distribution because of its plasticity, and because its representatives are found 

 in widely varying situations. It is to be regarded as the least xerophilous of 



12 Butler, Ormond, A study on gummosis of Primus and Citrus, with observa- 

 tions on squamosis and exanthema of the Citrus. Ann. Botany 25:107-153* P ls - 4- 

 1911. 



n Cannon, W A., The root habits of desert plants. Carnegie Institution of 

 Washington. Publ. No. 131. pp.96, pis. 23. 1911. 



