238 XIX. STRUCTURE OF THE THALLOPHYTA. 



another. The hyphae can indeed, in many cases, come into so 

 close a lateral union, that a tissue is produced, which, as pseudo- 

 parenchyma, delusively imitates the appearance of the parenchy- 

 matous tissue of the higher plants. This pseudo-parenchyma, 

 however, is a result of the coalescence of cell-threads, and not of 

 cell-division taking place in three planes. In order to study this 

 kind of structure, we take the fructification of a Hymenomycetous 

 fungus. We choose the common Mushroom, Agaricus (Psalliota) 

 campestris, because this fungus can now be obtained at any 

 season of the year, and shows, besides, a comparatively simple 

 structure. 



Agaricus campestris. We prepare first a delicate longitudinal 

 section from the stalk (stipe) of a fully-developed specimen. We 

 recognise clearly a structure of longitudinally-disposed hyphae, 

 and can readily tear the section in longitudinal direction with the 

 needles. The hyphae are directed more or less parallel to one 

 another ; occasional ones also run obliquely between the others. 

 Each hypha forms a septate thread, which is branched here and 

 there laterally. The branches arise either close under a septum, 

 or else lower down on the sides. Here and there we come across 

 a " blind " end of a branch. The cells of neighbouring hyphae not 

 infrequently appear connected by a horizontal branch, and have 

 open communication with one another. In the periphery of the 

 stipe the hyphae are narrower, and more closely pressed together ; 

 just under the surface the walls become brown, and their cavities 

 more or less completely collapse. Towards the middle of the 

 stipe likewise the hyphae become narrower, but their texture much 

 looser, and, their course quite irregular. Great masses of air 

 here fill up the interspaces between the hyphae. So long as the 

 destructive influence of water has not made itself felt upon the 

 contents of the hyphae, very little of these contents is to be 

 noticed ; only at the cross-walls they show, here and there, more 

 aggregated. Later on large vacuoles begin to form in the cells. 

 Here and there small crystals are met with. 



The cross-section of the stipe has a parenchymatous appear- 

 ance, which is only lost in the more median parts of the section, 

 where the hyphae also offer their side views. This pseudo-paren- 

 chymatous tissue appears as if composed of unequal, irregular 

 polygonal cells, which leave between them more or less numerous 

 intercellular spaces and gaps (Fig. 94). On careful examination 

 of the section, we notice close in the middle of many cells a 



