OMPHALIA FLAVIDA 



425 



over the surface of a leaf must strike many gemmae on their broad 

 under side instead of laterally and thus, perhaps, it may detach them 

 from their pedicels more readily than if their axes had been 

 perpendicular to the leaf surface. 



The Attachment of a Gemma to a Leaf. — When a gemma falls 

 on to a leaf, it usually settles on its slightly concave upper surface 

 and readily becomes attached to the leaf, presumably by means of 

 the mucilage in which the clavate palisade cells, which form the 



Fig. 214. — OmphaUa flavida (from Trinidad). A, a gemma ripe 

 and ready for dissemination, seen from above. It grew in 

 moist air. Short infection hyphae can be seen at its 

 periphery, but the central disc cells are free from hypliae. 

 B, part of a longitudinal section of a ripe gemma {cf. Fig. 

 205), showing clavate peripheral cells a embedded in a 

 gelatinous matrix, some of them extended into infection 

 hyphae b. Drawn by A. H. R. BuUer and Ruth Macrae. 

 Magnification : A, 86 ; B, 406. 



exterior layer of cells of the gemma, are embedded (Fig. 213, A). 

 Gemmae which have fallen on to a leaf are not easily detached from 

 the leaf ; for, when air was blown at them through the nozzle of the 

 glass tube already used to blow them from their pedicels, they 

 remained on the surface of the leaf and were not driven away from 

 it. It thus appears that the ripe gemmae are well adapted for dis- 

 persion and for attachment : the wind can easily blow them from 

 their pedicels and thus transport them from leaf to leaf ; but, when 

 once they have settled on a leaf, they resist the action of the wind 

 in removing them. Owing to the manner in which they attach them- 

 selves to a leaf, their chances of causing new leaf-spots are increased. 



