Development of the Sexual Organs. As has been stated in the preceding section, developing antheridia were commonly seen at the growing apices of the prothalli, but only in a very few prothalli did I find groups of young archegonia. In the older regions of the prothallus, where both antheridia and archegonia not infrequently arise singly amongst old organs, I did not find any in the earliest stages of development, though many of both kinds in later stages were to be seen. The fact that the apex of the prothallus is generally very broad militated somewhat against the study of the young developing organs, for transverse sections in this curving region of the prothallus-head cut Fig. 17.—Transverse section of main limb of prothallus in older region, showing portions of old sexual organs, also two fertilized archegonia. × 100. them often obliquely. However, I was able to obtain a fairly good series of both, although certain points must be left for a more complete study. Perhaps it would not be out of place for me to describe at this juncture the methods adopted for the preparation of my material for microscopic investigation. After the preliminary study and drawing of each prothallus as it was dissected out of the tree-fern humus, it was killed and fixed by immersing for twenty-four hours in a solution of chromo-acetic acid, the formula for which is that given by Chamberlain on p. 21 of his Methods in Plant Histology (3rd ed., 1915). This was found to answer quite satisfactorily so far as the more obvious histology of the prothalli and sexual
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