37^ 



THALLOPHVTES. 



(C. candidus)y or the terminal conidium of each chain gives rise to a germinating filament, 

 if it is capable of germinating, while the other cells of the conidial chain produce 

 zoogonidia (C. Portulacoc). After the swarming is finished the zoogonidia become firmly 

 attached to the cuticle of the host, invest themselves with a thin cell-wall, and, in the 

 case of Peronospora infestans, put out a delicate germinating filament directly into an 

 epidermal cell, piercing through its outer wall. After it has entered the cell (Fig. 

 1 80, H) this filament absorbs the whole of the protoplasm of the zoogonidium, and, 

 after piercing also the inner wall of the epidermal cell, reaches an intercellular space, 

 where the mycelium developes. The zoogonidia of Cystopus are firmly attached near the 

 stomata of their host, and push their germinating filaments into their orifices (Fig. 180, G), 



"^rrrt 



Fig. iZo.— Cystopus candidus. A branch of the mycelium growmg at the apex t with haustoria h between the paren- 

 chymatous cells of Lepidium sativum; B conidia-bearing branch ofjthe mycelium; C, D, E formation of zoogonidia 

 from the conidia; F zoogonidia germinating; G a zoogonidium germinating on a stoma; H zoogonidium oi Peronospora 

 infestans penetrating through the epidermis of a potato-stem (after De Bary, X 400). 



and thus find their way at once into the intercellular spaces. When the mycelium has 

 once obtained a footing in the parenchyma of the host, it continues to grow in it, and 

 finally often spreads through the whole plant, putting out its conidia-bearing branches 

 at various places in the stem, leaves, or inflorescence. In this manner the (unicellular) 

 mycelium of P. infestans can even hibernate in the tubers of the potato, to undergo 

 further development in the shoots in the following spring. The sexual organs of 

 the Peronosporeae are developed in the interior of the tissue of their host. Spherically 

 dilated ends of branches of the mycelium shape themselves into oogonia (Fig. i8r, ^, og)j 

 in each of which an oosphere is formed out of a portion of the protoplasm {B, os). 

 From another branch of the mycelium a branchlet grows towards the oogonium, 



