170 Scientific Proceedings, Royal Dublin Society. 



on its upper side by its attachment to the palisade cells, a downward and lateral 

 extension follows, which has, as one of its consequences, the upward rolling of 

 the margins of the leaflets. 



As is well known, I'olled leaves give the impression of being thicker than 

 normal ones ; and on measurement this is generally found to be the ease. It is 

 not invariably so, because it occasionally happens that the growth in length of 

 the palisade cells of affected leaves is interfered with, and the consequent 

 reduction may be of such a magnitude as more than to counterbalance the 

 extension of the spongy parenchj^ma. It does seem to be a general rule, 

 however, that the spongy parenchyma of diseased leaves is thicker than normal, 

 as is proved by comparing the thickness of this tissue (including the lower 

 epidermis, which does not vary) in similar leaves, diseased and healthy, of the 

 same variety ; or by determining what percentage of the whole thickness of the 

 leaf is made up of spongy parenchyma in the two cases. This percentage was 

 found to vary in mature healthy leaves between 47 and 54 per cent., while in 

 corresponding diseased leaves it varied between 57 and 74 per cent. It is 

 believed that this demonstrated expansion of the cells of the spongy parenchjona 

 in a downward direction must be accompanied also by a certain amount of 

 lateral extension, and this seems to lie confirmed by the apparentlj^ more rotund 

 shape of the cells and by a reduction in the sizes of the intercellular air-spaces 

 between them. 



In view of the obviously more compact and less elastic nature of the palisade 

 layer, to which the lower and less rigid tissue is firmly attached, the result must 

 be an upward rolling of the leaves, just as a pair of metal strips of which the 

 lower has the greater coefficient of expansion must, if firmly fastened together, 

 roll upwards with a rise in temiaerature. The fact that a similar distention of 

 the spongy parenchjana was observed, though not to such a marked degree, in 

 the "artificially" rolled leaves of healthy plants is further evidence in support 

 of this view. In the course of three determinations, each including five measure- 

 ments, it was found that in the "artificially" rolled leaves of these healthy 

 plants the spongy parenchyma and lower epidermis very regularly made up 

 57 per cent, of the total thickness of the leaf, while in corresponding normal 

 leaves this figure varied from 46 to 53 per cent., the average being just under 

 50 per cent. Similar measurements made after the "artificially" rolled leaves 

 had become normal showed that the spongy tissue had i^ractieally returned to 

 its original size, the percentage it then formed of the total thickness of the leaf 

 being 51, as against 50 per cent, in the case of similar leaves which had never 

 rolled. 



Excess of starch in the leaves not confined to leaf-roll. — There is still 

 further evidence to connect the rolling of the leaves with the presence in them 

 of an excessive amount of starch. Rolling of potato leaves, as is well known, 

 is not confined to the leaf -roll disease. It may accompany other diseases, and 

 may also occur as a result of mechanical or other injury to the plant; and in 

 some at least of these cases the rolled leaves contain an excess of starch. This 

 was proved to be the case in the upper rolled leaves of certain stalks of healthy 

 plants of the variety Barley Bounty, which were partially broken across at the 

 base, where aerial tubers were being produced in the leaf axils just above the 

 injury. The remaining stalks and their leaves were normal in appearance. 

 When such rolled and normal leaves were kept in the dark with their 

 petioles standing in water for forty-two hours the normal leaves were found to 

 be practically free from starch, except for a little, principally along the basal 

 margins of the lower leaflets. The leaf-blades and petioles of the rolled leaves, 

 however, contained obviously more starch at this time. It was estimated that. 



