[Illustration: FIG. 33.--_A_, leaf of pig-weed (_Amarantus_), with spots of white rust (_c_), one-half natural size. _B_, non-sexual spores (conidia). _C_, the same germinating. _D_, zoöspores. _E_, germinating zoöspores. _sp._ the spore. _F_, young. _G_, mature sexual organs. In _G_, the tube may be seen connecting the antheridium (_an._), with the egg cell (_o_). _H_, a ripe resting spore still surrounded by the wall of the oögonium. _I_, a part of a filament of the fungus, showing its irregular form. All × 300.] Under normal conditions the spores probably germinate when the leaves are wet, and the filaments enter the plant through the breathing pores on the lower surface of the leaves, and spread rapidly through the intercellular spaces. Later on, spores of a very different kind are produced. Unlike those already studied, they are formed some distance below the epidermis, and in order to study them satisfactorily, the fungus must be freed from the host plant. In order to do this, small pieces of the leaf should be boiled for about a minute in strong caustic potash, and then treated with acetic or hydrochloric acid. By this means the tissues of the leaf become so soft as to be readily removed, while the fungus is but little affected. The preparation should now be washed and mounted in dilute glycerine. The spores (oöspores) are much larger than those first formed, and possess an outer coat of a dark brown color (Fig. 33, _H_). Each spore is contained in a large cell, which arises as a swelling of one of the filaments, and becomes shut off by a wall. At first (Fig. 33, _F_) its contents are granular, and fill it completely, but later contract to form a globular mass of protoplasm (G. _o_), the germ cell or egg cell. The whole is an oögonium, and differs in no essential respect from that of _Vaucheria_. Frequently a smaller cell (antheridium), arising from a neighboring filament, and in close contact with the oögonium, may be detected (Fig. 33, _F_, _G_, _an._), and in exceptionally favorable cases a tube is to be seen connecting it with the germ cell, and by means of which fertilization is effected. After being fertilized, the germ cell secretes a wall, at first thin and colorless, but later becoming thick and dark-colored on the outside, and showing a division into several layers, the outermost of which is dark brown, and covered with irregular reticulate markings. These spores do not germinate at once, but remain over winter unchanged. [Illustration: FIG. 34.--Fragment of a filament of the white rust of the shepherd's-purse, showing the suckers (_h_), × 300.] It is by no means impossible that sometimes the germ cell may develop into a spore without being fertilized, as is the case in many of the water moulds.