276 DIVISION 77. COURSE OF DEVELOPMENT OF FUNGI. 



one another without interstices, the uppermost inclining towards one another and 

 closing over the apex of the spore-chains ; and as this is done from the first, before 

 the hymenial layer has reached its definitive breadth, the envelope is formed at least 

 at the same time as the first basidia, and perhaps before them. All cells of the 

 envelope are polyhedric in form, and are distinguished from the spores by their 

 larger size, by their thicker wall which often shows a very delicate prismatic structure, 

 and by their slightly granular or quite pellucid contents, ultimately often containing 

 air. The spore-layer consisting of the spore-chains with the envelope increases in 

 circumference by the constant introduction of new elements from the base and their 

 subsequent enlargement, and encroaches on the surrounding pseudo-parenchyma. 

 Its increase in breadth squeezes the adjacent cells together till they can often be 

 no longer recognised. By its growth in length the summit of the perithecium 

 is first pierced, the epidermis of the host is ruptured, and the spore-layer is 

 raised with the summit of the perithecium above the epidermis and grows, if pro- 

 tected from injury, by constant additions from below into a tube filled with spores 

 and upwards of i mm. in length. After it has burst through the epidermis, the 



cells of the envelope separate from one another 

 at the apex, and the envelope itself opens out 

 into the shape of a cup (Fig. 124 la), or in 

 some species (Gymnosporangium Sabinae) is 

 split longitudinally into narrow lobes ; the upper- 

 most ripe spores fall out, and this disintegration 

 of the envelope and the spore-chains advances 

 in the direction of the base, more rapidly in 

 exposed specimens and when the moisture of the 

 surrounding atmosphere varies than in cultivated 

 d careful] y Protected plants. 



The exceptional case mentioned above is 

 matia. a magn. , * about 350 times. exemplified in the aecidia of the genus Phrag- 



midium, which are distinguished from all others 



in the mature state by having no compact tubular envelope. In place of an envelope 

 a circle of club-shaped unicellular hairs, paraphyses, occupies the margin of the 

 hymenia, which often spread out into broad cushion-like layers; their earlier de- 

 velopment has not been investigated. 



Except in a very few cases the aecidia are always accompanied by spermogonia, 

 which are in all points very like the more simple organs of the same name described 

 above in Collema and other Lichen-fungi (Figs. 1247 sp, 126). In most species they 

 are small roundish to flask-shaped receptacles, looking to the naked eye like dots 

 sunk in the subepidermal tissue, and have a thin smooth wall formed of several 

 layers of closely woven hyphae which at length opens in the epidermis. The wall 

 encloses a single cavity, and closely packed sterigmata springing from the whole of 

 the inner surface of the cavity converge towards its centre leaving a narrow space free, 

 which is afterwards densely filled with spermatia. A few rows of narrowly subulate 

 pointed paraphyses or periphyses grow from the mouth of the receptacle instead of 

 the sterigmata, and pierce through the epidermis and issue to the open air, forming 

 a slightly divergent tuft, in the middle of which is the very narrow canal of the mouth. 



