2G4 BOTANY. 



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

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

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

 baffled the most skilled botanists, 'i'he two genera Peronospora and 

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

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

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

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



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

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

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

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

 oospores may be found in abundance in the dried and shrivelled parts of 

 the att'ect.ed leaves.* P. parasitica is common in spring and early sum- 

 mer, on Cruciferse, especially on Lepidium, Oapsella, Draba, etc., Ire- 

 quenlly 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 tlie leaves, stems, and even tlie tubers. Other species occur 

 on Eupatorium, Bidens, Ambrosia, Impatienx, Poteniilla, Anemone, 

 etc. 



(c) Tlie species of Oystopus 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 OapseUa and other Cruciferae ; and C. Bliti com- 

 mon on Porlulaca oleraaea and species of Amarantus in summer and 

 autumn ; tlie 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 ot potasli, and then, while the preparation is hot, a 

 considerable quantity of acetic acid should be addea ; the effervescence 

 which follows separates the softened tissues so that but little difficulty 

 is experienced in isolating large portions of the mycelium witli 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 rucACEiE. 



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 fungussee a paper " On the American 

 Grape-vine Mildew," by Professor W. Ot. Farlow, in Bulletin of the 

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

 described. 



