THE THALLUS 5 



ectoparasitic, less often in parasitic, forms it may cling fast to the sub- 

 strate by holdfasts known as appressoria (Fig. 215, 2). Usually it is 

 able to absorb food over its whole surface; yet for the better fulfilment of 

 this function, special hyphae or hyphal branches are developed in sapro- 

 phytic forms as rhizoids (Fig. 55, 1) and in parasitic forms, haustoria 

 (Fig. 120). Occasionally these structures function as holdfasts as well 

 as food absorbers. It is still an open question whether the haustorium 

 is a normal organ or whether it is not more often restricted in growth and 

 deformed by the action of the host cells. 



In many cases the hyphae grow together in groups, intertwine, adhere 

 and form a thick tissue which is called plectenchyma. If the single 

 hyphal elements are still recognizable as such (Fig. 2, 6), they are called 



Fig. 2. — Claviceps purpurea. Section through a sclerotium. 

 parenchyma; b, core tissue of prosenchyma; r, rind. 



o, peripheral layer of pseudo- 

 (X 360; after Tavel.) 



prosoplectenchyma or prosenchyma; if the hyphae have lost their indi- 

 viduality so that they lie beside each other (in sections) with the cells 

 appearing isodiametric and continuous, as in the parenchyma 

 of higher plants (Fig. 2, a), they are called paraplectenchyma or 

 pseudoparenchyma. 



In sclerotia the plectenchyma appears tuberiform with a firmer 

 pseudoparenchymatic rind and a looser prosenchymatic core. This 

 structure serves to carry the organism over unfavorable conditions of 

 growth and, with the return of normal conditions, germinates to the usual 

 mycelium or to a fructification. Bulbils are small sclerotia formed of a 

 few layers of cells, and are often present in large numbers. 



Rhizomorphs indicate a further step in the development of plecten- 

 chyma. They arise chiefly from parallel hyphae and often have a definite 

 apical growth from an apical meristem, as the root tips of cormophytes. 

 Under suitable conditions, they may again spread out in sheets of myce- 



