438 



formed of four or five rows of colorless cells with very wide lumina but 

 poor in contents (d). The underlying palisade parenchyma, extending into 

 the hypoderm in the conical-like buttresses (e) is usually not affected, but, 

 like the spongy parenchyma, poor in chlorophyll, often exhibits strongly 

 refractive, often green colored drops in its cells where the cork is formed. 



Often such corky masses ver}^ greatly resemble certain fungous dis- 

 eases as I have had opportunity to observe in Pelargonium sonale. 



The under sides of the leaves were covered with white cystopus-Iike 

 masses, isolated or united into large groups. These were hemispherical 

 cork excrescences, later separated from one another like fans and filled 

 with air. They began with an enlargement of the spongy parenchyma, 

 whereby all the intercellular spaces were filled up. The epidermis, as a 

 rule, remained unchanged while the mesophyll cells adjoining it were elon- 

 gated perpendicularly and were divided by cork walls, with a gradual loss 

 of chlorophyll. The cork cells partially lost their parallel arrangement 

 because of an irregular increase and were much distended until the epidermis 

 ruptured. The epidermis, however, manifested its restraining influence by 

 pressing upon the cork cells, so that their walls seemed crumpled. The 

 process of elongation and suberization extended deeper and deeper into the 

 mesophyll until at times the excrescence was four times as thick as the leaf. 

 A brown, twisted mycelium (possibly a Cladisporium) grew into the stomata 

 and later into the wound of the rupturing cork excrescence. 



Grapes are especially susceptible to intumescences and especially those 

 plants taken from greenhouses into the open for early forcing. In addition 

 to the excrescences on the leaves, little knots were formed on the stem of 

 the grapes, and, since the structure of these differed from the warts already 

 described, they may be considered here more thoroughly. 



Fig. yy is a cross-section through such a knot. The vascular bundles, 

 forming the wood-ring of the stem, are indicated by h, the pith by m; the 

 hard bast by hh ; the abnormal change in the bark parenchyma extends to 

 this point. This change is characterized by a distension of the parenchyma 

 lying underneath the collenchyma-like elements and an ultimate elongation, 

 the cells of which have subsequently divided. Because of this over elon- 

 gation the collenchyma {c) is pressed together and, without previously 

 having participated in the elongation, dies together with the epidermis. The 

 normal epidermis may be recognized at e; k indicates the cork zone formed 

 on the boundary of the dying tissue. The latter may not always be found, 

 however. Often the dying tissue passes over imperceptibly into the very 

 thin-walled, still living tissue w^hich shows slight cork formation at the place 

 of transition, eg indicates the normal collenchyma, occurring in groups and 

 not in connected rings. The division and over-elongation of the bark 

 parenchyma and the absence of cork excrescences distinguish these knot- 

 like intumescences from the cork warts which, in an immature stage, 

 resemble them greatly. 



