1907] BU RUN GAME— SPORANGIUM OF OPHIOGLOSSALES 



49 



r 



accurate means of estimating the decreasing density of their cytoplasm. 

 At about the time the spore coats attain a sufficient rigidity to resist 

 collapse, the cytoplasm begins to increase in density (^^. 27). Soon 

 afterward {-jig. 28) .starch makes its appearance in the spore. About 

 the same time the spore coat can be seen to consist of an exospore and 

 an endospore. As the spore becomes older the two regions of the 

 wall become more distinct and the exospore increases considerably 

 m thickness. In the mature spore there are two cytoplasmic zones, 

 an outer less dense one and an inner dense one filled with starch. 



Whether 



m 



brane which has differentiated into two could not be determined in 

 Ophioglossum. Appearances, however^ favor the latter view. Per- 

 haps the strongest evidence in its favor is the fact that though the 

 endospore is less distinct when first distinguishable, it is just as thick 

 or even thicker than in the mature spore. Somewhat various opinions 

 have been held concerning the exact method of exospore formation. 

 DeBary ('64) states that the exospore of certain Ascomycetes is laid 

 down from the epiplasm, which might be thought to correspond 

 roughly with the tapetal plasmodium in respect to the part either 

 might play in the formation of the outer coat of the spore. The fact 

 that the entire tetrad is surrounded for a considerable time by thie per- 



* 



sistent mother-cell wall, and that the whole lies in a vacuole, would 

 seem to render this hypothesis untenable in respect to Ophioglossum. 

 Fitting(:oo), Beer (:04), and others maintain that the exospore, 



m 



. space containing no protoplasm, can be built up by its activities. 



Miss Lyon ( : 05) has shown that the two coats in Sclaginella are 

 formed by the differentiation of a single thick homogeneous gelati- 

 nous membrane. The exospore according to this view is simply the 

 outer part in which has been deposited dense granular material ; the 

 roughness is due to the irregular way in which the material is depos- 



ited: 



deposits 



made. This explanation seems to harmonize with the facts in 

 Ophioglossum. Miss Lyon points out that the membranes of Sclagi- 

 nella are not separated and suggests that where they seem so it is due 

 to action of the reagents used in preparing the material. There is 



Mo Bot.Garaen 



190S 



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