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, 



