THALLOPHYTA. 



669 



Peronosporece. — Are mostly parasitic upon Flowering Plants, and the cause of 

 many destructive diseases. They establish themselves by means of a branching, 

 tubular, non-septate mycelium which penetrates the intercellular system of the host- 

 plant (cf. p. 56). They propagate asexually by means of unicellular sporangia borne 

 on branched hyphse which project from the stomates, &c., of the host {cf. fig. 381^); 

 these sporangia (or spores as they are sometimes termed) liberate on a moist sub- 

 stratum a number of swarm-spores (figs. 381 ^•■^■•') which originate new plants. 



Fig. 3S0.— Swann-spores in Saprolegniacese and Chjtiidiace». 



' Achlya prolifera. 2, s. » Successive stages of swarm-spore-formation in Achlya proli/era. ' Chytridium Olla, paiasitic on the 

 oogonium of the Alga aidogonmm ; development of swarm-spores. « Saprolegnia lactea. ' Development of swaim-sporea 

 in the same. (Partly after De Eary and Pringsheim.) ' x 20 ; 2, », • x 400 ; 6 x 300 ; « x 100 ; ' x 300. 



Sexual reproduction also takes place by the formation of oogonia and tube-like 

 antheridia. The latter become attached to the former (fig. 381^), and, putting out 

 fertilizing tubes which penetrate to the egg-cell within the oogonium, transmit their 

 spermatoplasm. No spermatozoids are differentiated, but the spermatoplasm travels 

 en masse. The fertilized egg-cell enters on a resting stage, and when it germinates 

 may either give rise to swarm-spores (e.g. Cystopus) or grow at once into a new 

 plant {Pythium, Peronospora). To Phytophthora infestans is due the well- 

 known Potato-disease. The Fungus attacks the foliage and reproduces abun- 

 dantly asexually. Later, its mycelium penetrates to the tubers and passes into 

 a dormant state there. Consequently when stored these potatoes go bad, and if 



