3;-2 



BOTANY 



Order 1. Exoasci 



a,. 



Of this order, which constitutes the simplest group of Ascomycetes, in which no 

 specially distinctive fructifications are formed, the most important genus is Exo- 

 ascus. The various species of Exoascus are parasitic on different trees, and as their 



mycelia hibernate in the tissue of the host- 

 plants, they cause a constant recurrence of the 

 same disease. The presence of the mycelium 

 in the tissues of the infected part causes the 

 abnormally profuse development of branches 

 known as Witches'-beooms. Exoascus Carpini 

 produces the abnormal growths occurring on 

 the Hornbeam ; Exoascus epiphyttus, those of 

 Alnus incana. Exoascus deformans attacks the 

 leaves of the Peach and causes them to curl. 

 Exoascus Pruni is parasitic in the young ovaries 

 of many species of Prunus, and produces the 

 malformation of the fruit known as "Bladder 

 Plums," containing a cavity, the so-called 

 "pocket," in the place of the stone. In the 

 formation of asci, individual cells of the copi- 

 ously-branched mycelium ramifying between the 

 Transverse epidermis and cuticle of the infected part be- 

 come greatly swollen. These grow into club- 

 shaped tubes, which burst through the cuticle 

 and, after cutting off a basal stalk-cell, are 

 usually converted into asci with eight spores 

 (Fig. 274). The numerous asci are closely 

 crowded together, and united into a hymenial 

 layer. In consequence of their increased turgor, 

 resulting from an excessive absorption of water, the ascus-tubes become ruptured 

 at their free extremities and eject the spores. 



The spores frequently germinate while still enclosed within the asci (Fig. 274, 

 a s , a 4 ), and give rise by budding to yeast-like conidia, e.g. Exoascus Pruni. 



The related genus Taphrina is parasitic on leaves, but its mycelium is not 

 perennial. Leaves infected with this Fungus assume a spotted, diseased appearance. 



Fig. 274. — Exoascus Ptiini. 



section through the epidermis of an 

 infected plum. Four ripe asci, ai, a-> 

 with eight spores, as, a± with yeast- 

 like conidia abstricted from the 

 spores ; st, stalk-cells of the asci ; 

 m, filaments of the mycelium cut 

 transversely ; cut, cuticle ; ep, epi- 

 dermis. (After Sadebeck, x 600.) 



Order 2. Perisporiaceae 



This order, which includes only Ascomycetes with enclosed fructifications, com- 

 prises three families : the Erysipheac or Mildew Fungi, the Perisporieae, and the 

 Tuoeraceae, Truffle Fungi. 



1. The ErysipAeae form a family of distinctive epiphytic parasites whose 

 mycelium, somewhat resembling a cobweb, and ramifying in all directions over the 

 surface, particularly the leaves, of higher plants, sends out haustoria which pene- 

 trate the epidermis of the host. The ripe ascus fructifications (Perithecia) are 

 black when mature and visible to the naked eye. In the simplest forms (e.g. in 

 the genus Sphaerotheca) the spheroid perithecium encloses only a single ascus 

 with eight spores. The ascus arises directly from the end of a fertile, ascogenous 

 hyphal branch after the septation of a stalk-cell. It is enveloped by a covering of 

 sterile hyphai produced by the mycelium at its base and forming a sheathing 



