188 PHYLUM IV. SIPHONOPHYCEAE 



branching tubes, whose cavities are continuous through- 

 out. They usually grow between the cells of their hosts, 

 and draw nourishment from them by means of little 

 branches fhaustoria), which thrust them- 

 selves through the walls. 



277. The asexual spores (conidia) are 

 produced upon branches (conidiophores) 

 which protude through the epidermis of 

 Fig. 78.— Piasmopara the host. In the Downy Mildews (Per- 

 onospora, Phytophthora, Piasmopara, 

 etc. ) these branches find their way through the breath- 

 ing-pores and bear their spores singly upon lateral branch- 

 lets; in the White Rusts (Albugo) the conidia-bearing 

 branches collect under the epidermis and rup- 

 ture it. Here the conidia are borne in chains 

 or bead-like rows. 



278. In some genera the relationship to the 

 Water Molds is shown by the- fact that these 

 conidia upon falling into water become true 

 sporangia, within which few to many zoospores 

 are produced. These after a free-swimming period be- 

 come motionless and germinate by means of a tube which 

 bores its way into the host. In two genera, however 

 (Bremia and Peronospora), the conidia themselves germ- 

 inate directly by a tube. 



279. The sexual reproduction takes place in the inter- 

 cellular spaces of the host. Lateral branches of two kinds 

 appear upon the hyphae; those of one kind (the young 

 oogones) become greatly thickened and finally assume a 

 globular shape; the other branches (the young antherids) 

 become elongated and club-shaped, both becoming sepa- 

 rated from the main filament by cross partitions. The 

 antherid comes in contact with the oogone which it 

 penetrates by a tube, through which fertilization occurs, 



