[Vol. 10 



116 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



usually develop most abundantly on the areas devoid of aerial 



mycelium. 



PYCNIDIA 



Pycnidia form in great abundance on infected, dead or mori- 

 bund pods, stems, petioles, and infrequently on leaves. Their 

 position relative to host tissue is apparently dependent on host 

 anatomy. The cortex of the stem of soybeans consists of (1) 

 an outer portion comprising the epidermis and 2-5 layers of thin- 

 walled chlorenchymatic cells, and (2) an inner portion made up 

 of thick-walled sclerenchymatic cells arranged in broad strands 

 which are separated by relatively few thin-walled cells. The 

 pycnidial initials develop in this outer sub-epidermal layer of 

 thin-walled cells, and the developing pycnidium spreads out 

 laterally to form a lenticular fungal structure- Enlargement, 

 however, is usually more rapid in the longitudinal than in the 

 transverse direction of the stem, and as a consequence the pycni- 

 dium becomes boat-shaped rather than truly lenticular (pi. 9, 

 fig. 10). It is broadly elliptical in transverse section and pos- 

 sesses a base flattened as a result of the resistance offered by the 

 underlying sclerenchymatic cells of the cortex to the centrifugal 

 pressure of the developing pycnidium (pi. 9, figs. 7-9). Subse- 

 quently, a very short beak forms, rupturing the epidermis and 

 affording a means of escape for the pyenospores. Occasionally, 

 a pycnidium may develop within or below the sclerenchymatic 

 cortical tissue. It then develops irregularly, obviously because 

 of inability to destroy or force these cells apart and make room 

 for normal enlargement. 



The anatomy of the pod wall permits a more nearly spherical 

 development of pycnidia. Immediately under the epidermis of 

 the pod there is a one-celled layer of sclerenchymatic tissue, and 

 at a distance beneath this is a second thicker layer of sclerenchyma 

 and vascular tissue, the two layers functioning to open the pod 

 upon its maturity. Between these inner and outer layers of 

 mechanical tissue of the pod wall is a several-celled stratum of 

 parenchymatic elements and scattered vascular bundles. Pycnidi- 

 al development occurs in this broad stratum of thin- walled cells, 

 and as little pressure is needed to rupture the overlying tissues, 



