OBSERVATIONS ON THE GENUS PTTHIUM. 487 



largest and most vigorous forms, but also on account of the 

 ease with which it may be obtained and cultivated, and further, 

 on account of its importance as a parasitic enemy of food and 

 other plants. It is, moreover, in every sense typical. 



Pythium De Baryanum.^ 



In almost any sowing of ordinary cress (Lepidium 

 sativum), certain of the seedlings after three or four days of 

 growth especially if kept moist may be observed to become 

 weak at that part of the stem which joins the root, and to bend 

 sharply over. In many cases the general rotting and death 

 of the seedling follow. An examination of such a diseased 

 seedling shows the part of the stem nearest the ground to 

 be in a rotten state. Instead of being turgid, semi-translucent 

 and of a pale greenish colour, the affected part is seen to be 

 much contracted; its cells turn brown or yellow, and become 

 unfit to support the weight of stem, &c., above it. At fig. 1 

 is a drawing of a seedling in this condition, straightened out 

 and with the soil washed from the roots. The line may be 

 taken to illustrate the level of the soil at the region where the 

 damage occurs. 



This rotting tissue, and parts of the stem immediately above 

 it, are seen under the microscope to be full of very delicate 

 hyphse, branching in all directions in and between the cells. 

 Closer examination shows that the thin-walled, cylindrical 

 hyphse are confined to the parenchymatous tissues, and avoid 

 the still young fibro-vascular bundles forming the more central 

 parts of the hypocotyledonary stem of the seedling. It is in 

 consequence of the flexible axial portion being no longer 

 supported by the turgid parenchyma, that the " top-heavy " 

 seedling bends sharply over at the injured spot. 



More minute examination shows that the hyphae run 

 through the tissues, especially in a longitudinal direction, and 

 if observation is particularly directed to the portions which are 



> Literature: — Hesse, loc. cit., 1874; De Bary, 'But. Zeit.,' 1881; and 

 •Beitrage z. Morph. u, Phys. d. Piize,' 1881, K. iv. 



