' DAMPING-OFF ' 695 



set free by the rupture of the enclosing bladder (d, Fig. 



233)- 



Each zoospore is a small naked piece of protoplasm possess- 

 ing two hair-like a'/ia (e, Fig. 233) by the movement of which it 

 is able to move about in drops of water. After swimming for a 

 short time the zoospore loses its cilia, rounds itself oif, and then 

 develops a delicate germ-tube capable of penetrating into the 

 tissues of plants where it soon grows into a new mycelium. 



These asexually-produced conidia, and sporangia arise in great 

 numbers, and by their immediate germination are capable of 

 spreading the fungus at a rapid rate. 



In addition to the above asexual methods of propagation, a 

 process of sexual reproduction occurs in this species of fungus. 

 The female reproductive organ, termed an oogonium, is spherical 

 {og, Fig. 233), and resembles a conidium or sporangium in shape. 

 A certain portion of the protoplasm within it collects in the 

 centre and becomes surrounded by a delicate membrane ; this 

 is the ovum, egg-cell, or oosphere of the oogonium, the rest of 

 the protoplasm within the latter being termed the periplasm 

 {p, Fig. 233). The male reproductive organ or antheridium 

 (a) is a long, somewhat large cell cut off from the end of a lateral 

 hypha arising near the oogonium. After coming in contact 

 with the latter, the antheridium develops a delicate fertilisation- 

 tube {£) which forces its way through the wall of the oogonium 

 and, finally, reaches the ovum. A small portion of the proto- 

 plasm of the antheridium is then transferred to the ovum, after 

 which act the latter develops a thick, external, brown, smooth 

 coat at the expense of the surrounding periplasm : the fertilised 

 ovum is then termed an oospore {s). The oospore is set free from 

 the enclosing oogonium and, after a resting-period of several 

 months has elapsed, it germinates and produces a germ-tube which 

 penetrates into any seedling with which it comes in contact 



Although thick-walled conidia may remain dormant in the 

 ground for some time and then germinate and reproduce the 



