364 BOTANr. 



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

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

 ever, much more difficult to follow, and iu some species it has thus far 

 baffled the most skilled botanists. The two genera Peronoapora and 

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

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

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

 tlirough the epidermis of the host (B, Fig. 176). 



(6) Several species of Peronospora are very easily obtained. P. mti- 

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

 shoots of tha grape ; Irom it may be obtained in midsummer an abun- 

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

 oospores may 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 Cruciferae, especially on Lfpidium, Capsella, DrcU>a, etc., fre- 

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

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

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

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

 on Ehipatorium, Bidens, Ambrosia, Impatiens, PotentUla, Anemone, 

 etc. 



(c) The species of Gystopus which are most common are G. eandidus, 

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

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

 mon on Porlulaea oleraeea 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 Portulaea. 



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

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

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

 considerable quantiiy 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. Q. Farlow, in Bulletin of (hi 

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

 described. 



