180 PHYTOPHTHORA PARASITICA. 



hangs down from its point of attachment. The disease spreads from 

 the leaf to the petiole, thence to the stem and the growing point, 

 killing the seedling. In older plants the disease in the field is 

 localised on the lamina or leaf -blade. As the diseased spot gets older 

 it turns yellowish and then brown. It spreads concentrically, and in 

 old spots forms concentric brown rings enclosing lighter brown areas 

 (Plate I). They are surrounded on the upper surface by a yellowish 

 green or dull pale green ill-defined border. On the under surface 

 the old spots have a brownish grey and ill-defined border which is 

 relieved occasionally with very minute white specks. The brown 

 central portion of the spots is always very sharply contoured. The 

 diseased areas either remain isolated or become confluent, often 

 covering almost the whole of the leaf. In case of confluent spots the 

 successive periods of their growth are not marked by concentric rings 

 but by wavy margins of different shades of brown. The spots are 

 not limited by the main ribs, even in the case of big mature leaves. 

 The diseased leaves have a tendency to fall off prematurely. Grow- 

 ing points and leaf-buds of plants of all ages are protected from the 

 external invasion of the disease by the waxy sheathing stipules 

 which completely cover them. Petioles of big leaves, stems with 

 thick epidermis, flowers and fruits, have been found till now in 

 nature to be immune from the ravages of the fungus. 



On the under surface of the leaf is seen with the aid of a pocket 

 lens a sparse white web of threads originating from the unhealthy 

 green coloured portion of the diseased spot. These are the long 

 and unbranched sporangiophores, measuring from 35 to over 500,u, 

 as a rule 100 to 3 OOm. They emerge from within the leaf as a rule 

 through stomata or between the cell walls of two contiguous 

 cells (Plate II, Fig. 1). They have been occasionally found to make 

 their way out by breaking through the upper wall of the epidermis. 

 They make their exit from the leaf singly, rarely in twos or threes 

 but never in clusters. 



In the petiole and stems it is a purely physical matter for the 

 hypha to make its way out of the tissues. Just underneath the 

 upper wall of the epidermis the hypha swells into a globular head and 



