OOPHTTA. ^ 143 



for Pond-scum Parasites. They are usually most abundant in -water 

 ■which has been standing for some time. Gall-fungi may be found 

 upon the leaves of Evening Primroses, Plantains, Mints and some 

 leguminous plants. In the study of these minute plants consult 

 the 45th, 46th, and 47th parts of Rabenhorst's Kryptogamen-Flora, 

 1892. 



(/) In the spring the leaves of shepherd's purse and peppergrass 

 may often be found covered underneath with a white mould-like 

 growth (Peronospora parasitica). Carefully scrape off a little of this 

 growth and mount first in alcohol, afterwards adding a little potassic 

 hydrate. The irregularly branching hyphse will be seen to bear here 

 and there their white, broadly ellipsoidal conidia. Similar studies 

 may be made of the Grape-mildew (Peronospora viticola) on grape- 

 leaves in autumn, and the Lettuce-mildew (Peronospora gangliformis) 

 on cultivated and wild lettuce from spring to autumn. 



{g) Make very tliin cross-sections of a leaf affected with a Mildew, 

 when the latter has passed the period of its greatest vegetative ac- 

 tivity. Mount in alcohol (to drive out air-bubbles), then add potassic 

 hydrate, and look for the resting spores, which in some species, are 

 of a dark-brown color. 



(Ji) White Rusts occur on many plants; one (Cystopus candidus) 

 on shepherd's purse, peppcrgrass, radish, etc. ; another (0. bliti) on 

 amarantus; and another (C. portulacae) on purslane. For conidia, 

 make very thin cross-sections of leaves, through a white-rust spot, and 

 mount as above. The resting spores (which are dark brown) are 

 easily obtained in the leaves of amarantus and purslane. 



Class IV. Fucace.^ {the Hockweeds). 



295. The plants of this class are entirely marine. In 

 many cases the development of the plant-body is unusually 

 perfect, showing a differentiation into parts which hare a 

 considerable resemblance to roots, stems, and leaves. In 

 size they approach the flowering plants. Their tissues, too, 

 show a high degree of differentiation; the cells are arranged 

 in cell-masses, and these are differentiated into several va- 

 rieties of parenchyma, approaching, in some instances, to 

 the condition which prevails in the Mosses and their allies. 



296. With the foregoing there is found a marked differ- 



