CHAPTER VII. THE THALLOPHYTA. 



;ii 



Fig. 509. — Mucor mucedo. a, a portion of vege- 

 tative mycelium, bearing an aerial hypha, termi- 

 nated by a sporangium, b ; b', sporangium more 

 highly magnified, showing the contained conidia ; 

 c, columella ; d, conjugating mycelial filaments, 

 with a zygospore formed between. 



The Entomophthorece, which are parasitic upon insects of 

 various kinds, are also probably to be classed here. Of these 

 Empusa Muscae, which infests house-flies, 

 and Entomophthora radicans, which often 

 destroys the larvae of the Cabbage-butterfly, 

 are common examples. 

 Some of the species have 

 been observed to form 

 zygospores. 



The Peronosporeae 

 are mainly parasitic on 

 terrestrial Phanerogams ; 

 a few, however, as some 

 of the species of Pythium, 

 are saprophytic, feeding 

 upon dead animal and 

 vegetable matters. They 

 produce unsegmented hy- 

 phae which ramify in the intercellular spaces of their hosts and, 

 sending haustoria into adjacent cells, absorb nourishment from 

 them. The great majority, after a time, send hyphae to the sur- 

 face, frequently out through the stomata of the plant, and these 

 bear sporangia which are shed, when ripe, in the same manner as 

 conidia, and hence are commonly so called, though they differ 

 from them in the fact that when they fall into water, the contents 

 break up into several rounded masses which escape as zoospores. 

 These, after finding a lodgment on the epidermis of the host-plant, 

 come to rest and produce hyphae, which either penetrate the walls 

 of the epidermal cells or find their way into the interior through 

 the stomata. A few of the forms, however, produce true conidia 

 which give rise to hyphae directly : a few others neither produce 

 conidia nor sporangia. 



Nearly all the species reproduce sexually by means of oogonia 

 and antheridia. The oogonium formed at the end of a hyphal 

 branch is similar in structure to the corresponding organ in the 

 oosporous Algae, but the antheridia consist of one or more slen- 

 der, curved out-growths, from the branch beneath the oogonium, 

 or sometimes from adjacent hyphae. In fertilization the anther- 

 idium applies itself directly to the surface of the oogonium, and 



