364 Journal of Agricultural Research voi. vi. no. 10 



K, L. In the latter case the fungus has penetrated the epidermis and the 

 external hyphse are sporangiophores. 



It is evident from a study of the sections that Rhizopus sp. does not 

 readily penetrate the unbroken epidermis from the outside. Hyphae 

 are found which extend for some distance along the surface of the berry 

 without penetrating. Plate XLIX, /, shows a portion of such a hypha; 

 even the germ tubes seem unable to penetrate readily and often grow 

 for some distance (PI. XLIX, M) over the surface without penetrating. 



Under field conditions or in moist chamber in the laboratory Rhizopus 

 sp. apparently very rarely enters the host cells. Although several 

 hundred slides were examined no single instance was found in which a 

 hypha had penetrated a cell wall. Plate XLIX, I-L, shows that the 

 hyphae typically grow between the cells along the middle lamella. The 

 effect of the fungus on the host cells is readily seen by the contraction 

 of the protoplasm. Plate L shows strawberry cells in various stages of 

 degeneration close to hyphae of Rhizopus sp. 



Plate L, A, shows the normal appearance of one of the smaller storage 

 cells of the strawberry. In this case the cytoplasm contains numerous 

 small vacuoles. Frequently, especially in larger cells, there is a single 

 large vacuole. Plate L, B, shows a similar cell in which the protoplasm 

 has begun to contract away from the wall. This cell was separated from 

 the nearest hyphae by three layers of cells. In Plate L, C, hyphae of 

 Rhizopus sp. are shown in contact with two host cells (a branch hypha 

 overlies one cell). The protoplasm of these cells is much shrunken, but 

 the cell walls retain their normal position, and the nuclei are unchanged. 

 Plate L, D, E, F, and G, show progressively later stages in the breaking 

 down of cells adjoining hyphae. In some (PI. L, D, F) the wall has begun 

 to collapse. In all except Plate XLIX, G, in which there was very 

 little cytoplasm remaining, the nucleus shows no signs of degeneration. 



This persistence of the nucleus in apparently normal condition after 

 the contraction of the protoplasm has progressed considerably is one of 

 the most striking characteristics in berries attacked by Rhizopus sp. and 

 is in sharp contrast to the condition found in berries rotted by Botrytis sp. 

 Often in a cell in which the cytoplasm has largely disappeared and the 

 wall is partly collapsed the nucleus appears large and typical, as in an 

 intact cell (PI. L, /). Frequently the cell wall collapses so rapidly that 

 no space is left between it and the contracted protoplasm (PI. L, H, I). 



EFFECT OF RHIZOPUS SP. ON BERRIES IN EXTREMELY DRY AIR 



In connection with experiments on the humidity relations of the fungus, 

 berries inoculated with Rhizopus sp. were placed in a desiccator with 

 concentrated sulphuric acid. Under these extremely dry conditions the 

 berry "leaked" in the characteristic manner, but the habit of growth of 

 the fungus was changed in two important particulars. • 



