449 



up to black spots or perforated the leaves. )If some leaves fell, when the 

 the plants were kept longer under the moist conditions, a great cushion of 

 intumescences was produced on the leaf scar which displayed similarity 

 to the wound callus. Older plants under similar conditions did not 

 develop intumescences as quickly, nor as abundantly, while very old leaves 

 developed none. Pieces of leaves, laid in moist cotton, after possibly two 

 days, were thickly covered with eruptions. Quickly growing plants react 

 most easily to the stimulus of a sudden change in the amount of moisture. 



These observations support our theory that the formation of intumes- 

 cences is the reaction of the organ to a stimulus due to a sudden increase 

 of atmospheric moisture. Only the immature organ reacts. If older leaves, 

 as we observed, for example, with Solanum Warscewiczii, respond with a 

 formation of intumescences after having been brought from the open air 

 into a damp greenhouse, these are exceptional cases of a special excitability 

 of the species. Such cases occur in various plant genera. 



My results differ from those of other investigators, since I always found 

 that intumescences invariably developed as the result of an arrested assimi- 

 lation due to an excess or deficiency of light. It always manifests itself, 

 however, in the scanty formation of solid reserve substances, usually, in fact, 

 those already formed become dissolved. In accord with Miss Dale's assump- 

 tion, the variation in assimilation may be connected with the increase of the 

 oxalic acid content in the cells showing in the abnormal increase in turgor. 

 In the same way, experiments with young leaves and pieces of leaves show 

 how the root pressure may be eliminated. 



Different combinations of the vegetative factors may give rise to that 

 deficient assimilation which shows itself in the formation of intumescences. 

 In the greater number of cases falling under my observation, I find the cause 

 to be an increase of heat and moisture given to a plant naturally dormant, 

 or being forced to arrest its assimilation from external conditions. The 

 following action throws light on inhibitory regulations. 



The Tubercle Disease of the Rubber Plant. 



On the under side of the leaves are found numerous abundant, very 

 small, gland or tubercle-like, hemispherical swellings. These are produced 

 by the pouch-like elongation (Fig. 87, int) of the cells of the leaf, which, 

 in a normal condition, have the form and arrangement shown at the side of 

 the picture marked ni and, therefore, are separated by larger or smaller 

 intercellular spaces (i). The morbidly elongated tissue (int) on the under 

 side of the leaf thus approaches the normal leaf pahsade parenchyma (/>) 

 which is provided with a three-fold epidermis {e). Of these three layers, 

 the outermost is small-celled and provided with a very thick layer of cuticle. 

 The innermost cell layer of the epidermis displays more thin-walled, com- 

 paratively very broad cells (w), which form the so-called water-storage, 

 protective layer. Isolated cells, enlarged like sacs, in this layer conceal 



