36 DISEASES OF TROPICAL PLANTS 



CH. 



palms, and other plants are frequently injured (see pages 

 106, 206). These semi-aquatic species reproduce non- 

 sexually by conidia and sometimes by spherical sporangia. 

 Zoospores are produced in both cases, and by this means 

 the fungus is enabled to travel over considerable dis- 

 tances. In sexual reproduction, enlargements known 

 as oogonia are formed. These are the female organs. 

 At about the same time the antheridia or male organs 



FIG. 22. Pythium Delaryanum, a fungus having aseptate mycelium during its 

 vegetative condition. Septa appear when the fruit is produced by which the 

 latter is cut off from the remainder of the mycelium. 



1, Seedlings of cress (lepnlium sntinun) attacked by the fungus; 2, mycelium bearing 

 conidia at the tips of the branches ; 3, sporangia of different ages, also a free zoospore ; 4, an 

 oosphere, with an antheridium or male organ, which has pierced the wall of the oosphere and 

 inserted a fertilising tube (after this blending of the contents of oosphere and antheridiinn, 

 the oosphere becomes surrounded by a thick wall, and becomes the oospore, or sexually formed 

 resting-spore) ; 5, a germinating conidium. All figs., except 1, magnified (after Massee). 



are formed near them. A tube from the antheridium 

 penetrates the oogonium and fertilization occurs, and 

 in due time an oospore or resting spore is formed. 

 Probably the most important species of this genus 

 found in tropical countries is P. Debaryanum, Hesse, 

 which attacks the seedlings of a very large number 

 of different host plants. Another very important species 



