THE VIRUS DISEASES OF PLANTS 425 



Passage of the Virus within the Plant 



So far the means by which the virus is conveyed from one 

 plant to another have been outhned. The next point of interest 

 is the consideration of the channels of carriage of the virus 

 within the plant. 



As already mentioned, practically the only gross changes of 

 a necrotic nature, such as might result from the localised action 

 of a parasite, that have been observed are in that part of the 

 vascular strand, the phloem, through which elaborated nutrient 

 material is carried. Even where no necrosis has been reported, 

 the translocation of starch is impeded in several cases, such as 

 peach yellows and spike disease of sandalwood, suggesting some 

 disturbance in the phloem. Other observations tend to support 

 the view that this is the main channel of extension within the 

 plant. As a rule the most certain method of transmission of 

 the disease from one plant to another is by the sucking insects, 

 chiefly aphids, which can convey it. It has been shown in 

 several cases that these insects feed by inserting their probosces 

 into the phloem, usually of the finer veins ; other tissues are 

 avoided, or tested and abandoned, until the phloem is reached. 

 Again in the cucumber mosaic about twice as many inoculations 

 succeed when the infective juice is pricked into the neighbour- 

 hood of the vascular bundles exposed when a leaf is torn oif than 

 when made into the stem at random. In the infectious chloroses 

 girdling experiments have demonstrated that the virus is carried 

 only through the bark, which in plants such as those worked 

 with includes the phloem tissues, but not the other elements of 

 the vascular ring. Furthermore, the symptoms after inocula- 

 tion always appear first in just those parts to which the flow of 

 the elaborated food is directed, namely in the young developing 

 shoots and the like. 



That the vascular system is in some way connected with 

 the passage of the virus is made more probable by the frequent 

 failure of the latter to reach the embryo. It is known that 

 there is no vascular connection between the mother plant and 

 the embryo, the bundles terminating either at the base of the 

 ovule or spreading around its seed coats, but never penetrating 

 the nucellus which surrounds the embryo-sac. Hence the 

 latter is nourished entirely by osmosis. The embryo developing 

 within the embryo-sac is still further removed from vascular 

 connection with the mother plant. In the Cucurbitaceee the 

 nucellus is provided on the outside of its epidermis with a well- 

 marked cuticle, and this epidermis persists in the ripe seed, 

 though the rest of the nucellus is absorbed. The stalk end of 

 ^.he ovule (the chalaza) is suberified. Thus the whole of the 



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