264 BOTANY. 



(a) The plants of this order are easily obtained, and so far as their 

 structure is concerned, are easily studied. Their development is, how- 

 evei, much more difficult to follow, and in some species it has thus far 

 baffled the most skilled botanists. The two genera Peronospora and. 

 Cystopus are distinguished by their conidia, which in the first are ter- 

 minal and single upon branches of the aerial hyphae (Fig. 175), while 

 in the second they are in moniliform rows upon hyphae which burst 

 through the epidermis of the host (B, Fijr. 176). 



(b) Several species of Peronospora are very easily obtained. P. viti- 

 cola, the American grape mildew, is common on the leaves and young 

 shoots of the grape ; from it may be obtained in midsummer an abun- 

 dance of conidia and conidial hyphae, and in autumn (October) the 

 oosporesmay be found in abundance in the dried and shrivelled parts of 

 the affected leaves.* P. parasitica is common in spring and early sum- 

 mer, on Cruciferse, especially on Ltpidium, Capsella, Draba, etc., fre- 

 quently clothing the leaves with a white, frost-like down. P. infestans, 

 the potato fungus, is common in many parts of the country on the 

 leaves and stems of the potato, sometimes causing great injury by de- 

 stroying the leaves, sterna, and even the tubers. Other species occur 

 on Eupatorium, Bidens, Ambrosia, Impatien*, Potentilla, Anemone, 

 etc. 



(c) The species of Cystopus which are most common are C. candidus, 

 which may be found in the spring and summer as white, blister-like 

 blotches on the leaves of Capsella and other Cruciferae ; and C. Bliti com- 

 mon on Portulaca oleracea and species of Amarantus in summer and 

 autumn ; the latter is an excellent species to study, as its oospores are 

 very easily found, especially in the stems of Portulaca. 



(d) In preparing specimens for the study of the sexual organs, small 

 portions of the tissues containing them should be boiled for a minute 

 or so in a solution of potash, and then, while the preparation is hot, a 

 considerable quantity of acetic acid should be added ; the effervescence 

 which follows separates the softened tissues so that but little difficulty 

 is experienced in isolating large portions of the mycelium with oogonia 

 and antheridia. It frequently happens that the parts are rendered 

 more distinct by the addition of iodine to the specimen after mounting 



IV. CLASS FUCACE^E. 



350. The plants of this class, composed of marine spe- 

 cies, present, in most cases, a development of the plant-body 

 which is unusually perfect for the Thallophytes. In many 



* For the best account of this fungus see a paper " On the American 

 Grape-vine Mildew," by Professor W. G. Farlow, in Bulletin of tlie 

 Bussey Institution,Vol. I., p. 415. Several other species are also briefly 

 described. 



