126 KANSAS UNIVERSITY QUARTERLY. 



ridges. For this reason the growth of the fungus is more circum- 

 scribed laterally and the mechanical injury to the leaf is much less 

 extended than is usually the case. Between the bundles are from two 

 to three layers of parenchyma cells, and in the figure the mycelium is 

 seen to permeate this tissue and form spore clusters on both surfaces. 

 After the spores break through the cuticle they extend out laterally so 

 that the sorus appears of considerable breadth. It is only in cross 

 sections that the narrow limits of the mechanical injury of the leaf 

 produced by a single sorus becomes evident. The section of a leaf 

 of Ave7ia sativa parasitized by Piiccinia coi'onata, lig. 2, plate XII, 

 is in striking contrast to fig. i. In the leaves of oats the paren- 

 chyma has a broadly lateral distribution and the bundles are for the 

 most part more deeply seated than in Spartiiia. For this reason they 

 do not always serve as a check to the lateral extension of the myceli- 

 um, and the sori may even become confluent over them, as shown in 

 the figure. The whole of the parenchyma in the region of the affected 

 parts is permeated by the mycelium, and ytX. the chlorophyll corpus- 

 cles remain in an apparently healthy condition. 



With these examples before us, we may say that the first visible 

 injury due to this class of parasites is mechanical ; the cuticle and in 

 some cases a portion of the parenchyma is broken and pushed out- 

 ward. The fibro-vascular bundles do not seem to be affected by the 

 parasite. The mycelium is seen to extend throughout the parenchyma; 

 but the tissues appear to be in a fairly normal, condition, except that 

 in many cases the cells have been crowded by the pressure of the 

 developing spores. Injuries other than mechanical and locally visible 

 must necessarily follow the presence of a parasite thus deeply seated 

 in the vital parts of a plant. The mycelium permeating the paren- 

 chyma cells is undoubtedly appropriating for the development of its 

 spores the food material elaborated by those cells, and the healthy 

 nutrition of the whole plant must be measurably restricted thereby. 

 Moreover, the undue evaporation caused by the rupture of the cuticle, 

 and the osmotic conditions produced by the rapidly developing spores 

 of the parasite must divert the salts brought up from the soil, and the 

 sap elaborated in other parts of the plant from their proper distribu- 

 tion ; and not only the leaves and culms where the parasite is local- 

 ized, but the roots and inflorescence must be impaired in their devel- 

 opment. The influence of the position and size of the fibro-vascular 

 bundles on the extent of the mechanical injury done by the parasite is 

 made evident by a comparison of the figures of plates XI and XII. 



Smut on corn is one of the most conspicuous of plant parasites at 

 the time of the formation of its spores. I presume nearly every farmer 

 boy has looked on its silvery masses with feelings not unmixed with 



