73 a 



MARSILEACE-E. [Acrogexs. 



Very full details respecting the structure of this order are given by Mr. Henfrey, 

 whose report is here quoted almost literally :— ,..-.. 



The spores of the Isoetes lacustris are of two kinds, analogous to those of the 

 Lycopods ; both kinds being produced in spore-cases imbedded in the bases of the 

 leaves but the large spores are found in great numbers, not merely four in a 

 sporangium as in the Lycopods. The development of the spores was little known 

 until the publication of an essay on the subject in 1848, by Dr. C. Miiller, forming a 

 sequel to his researches on the Lycopods. 



Miiller compares the. complete large spore, as discharged from the sporangium, to 

 the ovule of flowering plants ; and he describes it as a globular sac enclosed by three 

 coats, which he names the primine, secundine, and the nucleus. The outermost coat, 

 or primine, is stated to be composed of a thick cellular membrane exhibiting a raised 

 network of lines, which give it the aspect of a cellular structure, but are in reality 

 analogous to the markings on pollen-grains. The outer surface exhibits the lines 

 indicating the tetrahedral arrangement of the spores in the parent cell, as in 

 Selaginella, and it is at the point of intersection of these that the membrane gives 

 way°in germination. The next coat, or secundine, is another simple membrane lining 

 the first. The nucleus is a coat composed of delicate parenchymatous cells, but 

 among these are found groups of peculiar character. These are described as consisting 

 of a large cell divided by two septa crossing each other at right angles, projecting 

 from the general surface, being either oval in the general outline, or having four 

 indentations opposite the cross septa, so as to give the appearance of the structure 

 being composed of four spherical cells. The cells surrounding them are of irregular 

 form, different from the generally six-sided cells of the rest of the nucleus. Many of 

 these groups occur on the nucleus, always at the surface of the coat where the primine 

 and secundine afterwards give way, scattered without apparent order over it, but one 

 always near the point of the opening. To these structures Dr. Miiller did not 

 attribute any important function, explaining them merely as produced by peculiar 

 thickenings of the tissue to protect the pro-embryo during germination. The 

 contents of the nucleus were stated to resemble those of the cavity of the spores of 

 Selaginella. 



In these contents, which become dense and mucilaginous, a free cell is developed 

 near the upper part of the cavity ; this is the rudiment of the embryo, and by cell- 

 multdplication it becomesa cellular mass, which soon begins to exhibit growth in two 

 directions, producing the first leaf and the first rootlet, projecting from a lateral 

 cellular mass, which the author calls the " reservoir of nutriment." The embryo then 

 breaks through the coats, the first leaf above, and the first root below, the coats 

 remaining attached over the central mass of the embryo. The subsequent changes 

 need not be mentioned here, further than to state that the leaves succeed each other 

 alternately, and are not opposite, as in the Lycopodiacea? ; moreover, no internodes 

 are developed between them, so that the stem is represented by a flat rhizome, like 

 the base of the bulk of many Monocotyledons. 



In the paper by Dr. Mettenius, already alluded to, we find some very important 

 modifications of, and additions to, this history of development of the spores of Isoetes, 

 bringing them into more immediate relation with the other vascular Cryptogams. 



This author describes the spore-cell as a thick structure, composed of several 

 layers ; in some cases he counted four. It completely invests the pro-embryo, which 

 is a globular cellular body filling the spore-cell. Among the cells of the outermost 

 layer of the pro-embryo (which layer forms the nucleus of Dr. Miiller), on the 

 upper part, are produced the ovules, fewer in number than in Selaginella, arranged iu 

 three rows converging upon the summit of the spore, these rows corresponding to 

 the slits between the lobes of the outer coat of the spore. The four superficial cells 

 of the ovules (which are evidently the peculiar groups mentioned by Miiller, and 

 previously noticed by Valentine) grow much in the same way as in the Rhizocarpeae 

 and in Selaginella, into short papilla:. The embryo is developed in the substance of 

 the pro-embryo, displacing and destroying its cells, and a globular portion (cor- 

 responding to the "reservoir of nutrition" of Miiller) remains within the spore after 

 the first leaf and rootlet have made their way out. This body is the analogue of that 

 portion of the embryo of Selaginella which penetrates into the cavity of the spore, 

 and to the end of the first axis in the Rhizocarpeae. 



The most important point, however, of Dr. Mettenius's researches relates to the 

 phenomenon exhibited by the small spores. In the water in which the spores were 

 sown, he observed moving spiral filaments resembling those of the Ferns. He was 

 not able to trace all the stages of development of these spiral filaments from the 

 small spores, but he obtained nearly all the evidence relating to their origin which 

 Nageli has done iu reference to the similar organs in the Pilularia. In the small 



