554 



COMPARATIVE MORPHOLOGY OF FUNGI 



They develop as follows: The hyphae, which penetrate to the sur- 

 faces from the intercellular spaces intertwine to form a thin pseudoparen- 

 chymatous stroma. On its surface the hyphae grow perpendicularly 

 to the surfaces of the leaves or radially, toward a common center, 

 and form a layer of slender parallel hyphae (Fig. 371, 3) which 

 generally have terminal, somewhat annular, colored thickenings. 

 Above this ring, a young oval pycnospore develops. Meanwhile the 

 nucleus has divided, one daughter nucleus migrates through the 

 structure into the young spore which is separated above the thickening 



Fig. 371. — 1. Gymnosporangium clavariaeforme. Section of a pycnium on leaf of 

 Crataegus ( X 345). 2. Phragmidium violaceum. Pycnium on Rubus with ruptured cuticle 

 (X440). 3. Cronartium ribicola portion of pycnium (X 1,160). P, paraphyses; Ep, 

 epidermis; C, cuticle. (After Blackman, 1904, and Colley, 1918.) 



by a septum and which then falls off. The other remains behind in 

 the basal cell and divides repeatedly, forming many successive spores. 

 The periphery of the whole sorus is surrounded by a ring of periphyses 

 (in systematic works often called paraphyses) which have arisen by the 

 elongation of the outer cells of the stroma. They intertwine in a dome 

 over the fertile tissue and later by their pressure, rupture the epidermis. 

 In the primitive Pucciniastreae, the periphyses are absent, the cuticle 

 or epidermis of the host being the only covering of the sorus. At first 

 this is often penetrated by a small pore through which the pycnospores 

 (spermatia) are extruded, Later the whole cuticle or epidermis may 



