A PARASITE OF THE TREE FERN (CYATHEA) 



F. L. Stevens and Nora Dalbey 

 (with plates XV, XVl) 



Cyalhea arhorea (L.) J. E. Smith, one of the most beautiful of 

 the Porto Rican tree ferns, is usually heavily infected by black 

 fungous growths. Two collections of this fungus were made, one 

 at Maricao, July 19, 191 5, the other on El Alto de la Bandera, 



July 14, 1915- 



On the older leaves the fungus is so abundant that the smallest 

 frond segments, which measure about 3X7 mm., bear 25 or more of 

 the black spots, and no segment is free of the fungus. On com- 

 paratively young fronds infections are less numerous, but even on 

 such there are many fungous spots. A general idea of the appear- 

 ance of the disease may be gained from figs, i and 2. The spots are 

 often so abundant as to occupy considerably more than half of the 

 leaf area. The individual spots are irregular in outline, and slightly 

 elongated in a direction parallel with the veins of the host. The 

 center of the spot is occupied by a conidiiferous structure, oblong, 

 fiattish, and dimidiate. This opens by an irregular crack, and in 

 old pycnidia the whole top falls away (fig. 3). The cleavage lines 

 seem to be determined by irregular rows of large cells. Immediately 

 surrounding the pycnidium is seen a subiculum composed of close 

 h\^hae which appear to radiate much after the manner of the 

 Microthyriaceae (fig. 4). Close focusing shows that this layer, 

 instead of being superficial, is within the epidermal cells. Sur- 

 rounding this epidermal subiculum is an area in which mesophyll 

 cells alone are diseased. The diseased cells are dark brown and 

 are quite filled with the coarse dark mycelium, while the adjacent 

 cells are normal. In the cases of very young diseased spots, con- 

 sisting of only a few cells, the infection is entirely in the mesophyll 

 (fig. 19). It is only later that the central epidermal cells of a 

 spot become invaded. 



In microtome section these facts are verified: the mesophyll 

 cells are seen to be invaded first, later the epidermal cells. Then the 

 Botanical Gazette, vol. 68] [222 



