10 
H. MARSHALL WAKD . 
clication of tissue differentiation ; the cells of this outer layer 
only divide further by -avails perpendicular to the outer sur- 
face. The ovoid embryo now begins to have starchy and 
other grandular matter deposited in its cells, and becomes 
thereby too opaque for observation of the cell-walls until sub- 
mitted to the action of warm potash and glycerin or other 
clearing reagents. From the first no trace of vacuoles occurs, 
hut the cells are tightly packed and full of fine-grained pro- 
toplasm, with large bright nuclei, and thin firm cell-walls 
surround them. By proper treatment, however, one recog- 
nises in the last stages of the embryo (fig. 31) that yet 
another series of tangential walls has appeared concentric to 
the first, and thus the ovoid mass presents a central column 
of cells, surrounded by a layer one cell thick, while over all 
is another layer, also one cell deep. 
About this time the cells are crowded with nutritive 
matters, the pro-embryo and integument cells are empty and 
shrivelled (fig. 82), the remains of nuclei appearing on their 
dark brown walls, and the seed may be considered ripe. 
In reviewing the processes above described, we may shortly 
point out several views held of late as to their meaning, and 
to render this more clear and complete, it may be advisable 
to recall to mind some points more fully dealt with in the 
larger text books.^ 
In the Ferns generally we haVe a spore developing a free 
chlorophyll-bearing, and often large prothallus, on which 
are produced antheridia and archegonia. When, as in Os- 
munda, the prothallus only bears antheridia at times, we 
may consider this the carrying to a step further a process 
common to this and many other genera, where the arche- 
gonia appear later than the very numerous antheridia. If 
we suppose the appearance of the archegonia indefinitely 
postponed the prothallus becomes unisexual — male ; if pro- 
longed after all the antheridia have decayed the prothallus 
is practically female in function. 
Such a unisexal prothallus may be supposed diagramma- 
tically represented in Fig. U as a section passing through 
the germinating spore, prothallus, and archegonium. In the 
long free neck of the latter are seen several masses of pro- 
* Tlie English student has an excellent account in Sachs’ ‘ Text Book.’ 
Cf. also lloimeister, ‘ Higher Cryptogamia,’ Ray Soc., 1862, and Luerssen, 
‘ Med. Pharm. Bot.,’ B. i. Also literature quoted. 
2 Ti>e relative positions and sizes have not been insisted upon in the 
diagrams. In all the figs. Ex. = exospore, En. = endospore, Arch. 
points to neck of archegonium, and Can. to the canal-cells, Oos. = oosphere 
or egg-cell. 
