426 SCIENCE PROGRESS 



central part of the ovule is cut off from the seed coats and stalk 

 by cutin or suberin. The embryo also, in the spherical stage, 

 is provided with a complete investment of cuticle over its free 

 parts. All the evidence available in regard to infection by the 

 virus diseases indicates strongly that they are unable to pass 

 through cutin and probably unable to pass through suberin 

 {e.g. uninjured branches or a potato tuber). Hence one would 

 expect that the virus of cucumber mosaic would fail to be carried 

 over to the next generation in the seed. But this is one of the 

 diseases which is sometimes transmitted by seed. The explana- 

 tion of this anomaly is perhaps to be found in the work of 

 Longo,^ who has shown that since the cucurbit embryo is so 

 completely isolated by impermeable layers, an adaptation of a 

 curious nature has been developed in which the pollen tube 

 opens a communication between the neck of the nucellus and 

 the embryo-sac. Possibly the same channel could serve to 

 transmit the virus to the embryo. Other nutritional adapta- 

 tions for the absorption of organic food by the embryo, 

 haustorial organs of the most varied morphological value, are 

 known in a number of other families, and may perhaps have 

 some significance in the inheritance of some of the virus 

 diseases. 



On the other hand, Allard ' has shown that considerable 

 interruption of the vascular tissue does not appreciably increase 

 the time taken by the virus of tobacco mosaic (which is not 

 inherited) to pass from the leaf to the stem, and he is therefore 

 inclined to think it passes from cell to cell by diffusion, though 

 the fine anastomosing lateral veins may aid. Doolittle ' thinks 

 the latter are the more important in cucumber mosaic. 



In the infectious chloroses there is evidence that, though the 

 virus may be distributed throughout the plant, it only multiplies 

 in the chlorotic areas. If these are removed, as by defoliation or 

 carefully cutting out all the yellow spots on the leaves as they 

 appear, it is possible to get the plant to form only normal green 

 leaves ultimately. But the virus can persist in a latent form 

 in the dormant buds of such plants, and if these buds are forced 

 to grow, they give chlorotic shoots which reinfect the new 

 growth of the whole plant. The persistence of the virus in the 

 latent condition in perennial root stocks of plants like Phyto- 

 lacca and Physalis, in the swollen root of the beet, and in the 

 potato tuber, is well known. 



* Longo, B., Osservazioni e ricerche sulla nutrizione dell' embrione vege- 

 tale, Ann. di Bot., ii, p. 373, 1905. 



2 Allard, H. A., Further Studies of the Mosaic Disease of Tobacco, Journ. 

 Agric. Res., x, p. 620. 1917. 



' Doolittle, S. P., The Mosaic Disease of Cucurbits, U.S. Dept. of Agric. 

 Bull., 879, p. 39, 1920. 



