SPOROPHYTE. 33 



divide only transversely, and give rise to the three rows of cells of the 

 stalk (figf. 176). The four wall-cells form the walls of the sporangium. 



The archesporial cell at once cuts off four tapetal cells (fig's. 161, 162), 

 one on each of its faces, beginning- on the sides. Each tapetal cell divides 

 into four by anticlinal walls. The archesporial cell then divides nearly 

 vertically into two, and the tapetum divides periclinally into two layers 

 (fig-. 163). The exact relation in time of the divisions of archesporium 

 and tapetum is not constant (fig's. 164, 166). From this time onward the 

 archesporial cells divide exactly synchronously, though in different planes, 

 first into four (figs. 165, 166), then into eig-ht, and finally into sixteen. 

 As soon as the sixteen spore mother-cells are formed the tapetum begins to 

 degenerate (figs. 167, 170). Its walls dissolve, and the cytoplasm forms 

 a vacuolated mass, but the nuclei persist until the spores are acquiring 

 their definite walls (fig. 168). The spore mother-cells, at first in a solid 

 mass (fig. 167), enlarge rapidly and separate from one another. Their 

 nuclei especially increase to a relatively great size (fig. 170). The chro- 

 matin now lies in innumerable fine granules. Then a long, fine, and 

 tangled chromatin thread is organized, and synapsis ensues. This must 

 be a lengthy stage, as it is frequently and easily found. Emerging from 

 synapsis, a heterotypic division occurs. The two resulting nuclei at once 

 divide again, and spindle fibers are formed between all four of the daughter 

 nuclei. Across the fibers cell- walls are laid down. The spores remain 

 together in fours until their walls are thickened. They separate before 

 the final sculpturings are formed on the outside. Thus the spores always 

 show a tetrahedral angle. The sculpturings are irregular lines and lumps 

 of brown substance (figs. 174, 175). All stages in the formation of 

 sporangia and spores may be found in a single collection of material from 

 newly unfolded fronds (e. g., Loch Raven, Maryland, at base of a steep hill 

 facing north, June 4, 1905). Meanwhile the wall of the sporangium has 

 also reached its mature size and structure. The sides consist of very thin, 

 broad cells, about eleven on each side. The right and left sides are nearly 

 identical (figs. 171, 172). The annulus runs from the stalk up one edge 

 and over the top of the sporangium and about one-quarter of the way 

 down the other edge. Its cells, 18 or 19 in number, are cubical and 

 are heavily lignified on all except the outer walls. The mouth of the 

 sporangium, or point of rupture, lies between two long, narrow, slightly 

 thickened cells (sometimes two such cells on one or the other lip) about 

 midway between the end of the annulus and the stalk. Similar long, narrow 

 cells, three to five in number, but with thin walls, fill up the space above 

 and below the mouth. At maturity the whole sporangium and contents 

 dry out, and the spores are hurled away as described for another fern by 

 Atkinson (1894). The annulus tends to straighten, and it does so, slowly 

 opening the mouth of the sporangium and tearing the walls nearly straight 

 across their whole width. It bends far around backward until the head 



