52 A TEXT-BOOK OF BOTANY. 



loose tissue, more or less cellular in structure, that finally develops 

 into a yellowish leathery wall. This body, which may be regarded 

 as a closed ascocarp, is known as a perithecium (Fig. 27, F ; Fig. 

 28, F}. As a result of the conjugation of the fertile cells, asci 

 (Fig. 27, G, H, J; Fig. 28, G, //) develop within the perithecium, 

 which are more or less spherical or ellipsoidal and contain from 

 four to eight spores (ascospores) (Fig. 27, K; Fig. 28, H}. 

 After maturity the cellular tissue around the asci dries up and dis- 

 integrates, the walls of the asci dissolve, and the ascospores are 

 liberated from the perithecium by slight pressure. The spores 

 lie over winter and then germinate, producing a mycelium from 

 which conidia first develop and afterwards the perithecia, thus 

 repeating the life history of the plant. 



Ergot. Another Ascomycete of special interest is the fungus 

 known as Ergot (Claviceps purpurea). The spores of this 

 fungus germinate on the flowers of certain grasses. The myce- 

 lium penetrates the walls of the ovary, absorbing the nutriment. 

 After a time the mycelium develops on the surface, and from 

 this short conidiophores arise bearing small ovoid conidia (con- 

 idiospores) (Fig. 29, A}. The mycelium secretes a sweet fluid, 

 the so-called honey dew which attracts insects, and thus the 

 conidia are carried to other plants. As the conidia are capable of 

 immediate germination the so-called " ergot, disease " rapidly 

 spreads during the flowering season of the host plants. After the 

 formation of conidia ceases, the mycelium forms a dense mass 

 which is surrounded by a dark layer, and this, if developed upon 

 rye, constitutes the ergot grains (Fig. 29, B) used in medicine, 

 these grains being a number of times larger than the rye grains 

 which they replace. The mycelial tissues connected with the host 

 plant die, and the ergot drops to the ground. At this stage the 

 ergot mass is more or less cellular in structure and is known as the 

 SCLEROTIUM. It is quite resistant and usually remains dormant 

 until the following spring when the grasses are in flower again. 

 The sclerotium then shows signs of renewed activity by the de- 

 velopment of small, reddish, spherical bodies with a fair-sized 

 stalk (Fig. 29, C). Within the periphery of these spherical heads 

 are produced flask-shaped perithecia or ascocarps (Fig. 29, >) 



