BY ALEX. G. HAMILTOX. 773 



epidermis taken together are as thick as, or thicker than the layers 

 between. The epidermal cells are very clear and free from 

 contents. 



(3) The palisade-parenchyma composed of two rows of very 

 small oblong cells, the inner row smaller and rounder than the 

 outer and very closely packed. 



(4) A very dense spongy parenchj'ma, becoming more open 

 near the domatium. Both this and the palisade layer are very 

 dense all through the leaf and very full of chlorophyll bodies, so 

 that it is difficult e\en in the thinnest sections to make out the 

 structure. I found hydrate of chloral most useful in clearing the 

 sections. 



(5) The epidermis of the domatium, in two layers, the inner 

 composed of larger cells. 



(6) The inner cuticle, through which stomata open in all parts 

 of the cavity. The same layers occur in reverse order in the floor, 

 and running from the midrib and vein is an extension of the 

 round strengthening cells which occur outside these. 



The above is a description of the domatium in an ordinary 

 healthy state. I have rarely seen Acari in them. But some 

 time ago I came across a plant with very large domatia which 

 were evidently in an unhealthy state, being pale or brown, or 

 even black. On examining them, I found that all the unhealthy 

 domatia contained numbers of Acari and their ova. Sections of 

 these showed the palisade and spongy parenchyma cells greatly 

 swollen and very irregular in shape, and undistinguishable from 

 each other. Brownish patches occurred here and there, and also 

 in places a number of cells had taken a bright crimson colour. 

 In some of the cells of the mesophyll there was a deposit of 

 granular matter on the walls. The epidermal cells were normal 

 as to shape, but even larger than ordinary. Where ova rested 

 on the interior of the domatium, the cells were dark-coloured and 

 very closely placed. At the mouth, hairs of the same kind as in 

 Pennantta were placed. In three sections from the same domatium 

 I counted ninety-two ova. besides several young and mature Acari. 

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