LE 
DEVELOPMENT OF THE SPOROGONIUM IN THE MOSSES. 277 
there are four cells at the centre surrounded by eight cells. The 
exterior eight cells form the amphithecium of the theca (my exo- 
meristem), the four cells at the centre the endothecium (my 
endomeristem). This method of development is so far just the 
same for either the theca or seta. 
The terms exomeristem and endomeristem, first used by Russow 
(20) and Sanio (21), have been adopted by me in a slightly 
modified and restricted sense*. At the stage just described it 
will be seen that there are two sets of cells—the four central and 
the eight peripheral ones. Each of these sets henceforth grows 
according to a distinct. and different law. The four central cells 
and the meristem which they give rise to I have called the 
endomeristem ; and the eight peripheral cells and the meristem 
to which they give rise I have called the exomeristem. 
From the point just described above, I have observed the deve- 
lopment of the seta continuously up to the time when the theca 
begins to be formed. The development of the seta is acropetal ; 
so that a series of sections taken below one another illustrate 
the methods of development of the several tissues. 
The next stage of development shows that each of the exo- 
meristem-cells has been divided tangentially by a radial wall; so 
that the exomeristem consists of sixteen cells (fig. 35). So far, no 
divisions have appeared in the endomeristem. A little below 
the point shown in fig. 35 a ring of tangential walls appears in 
the exomeristem, which has therefore now become two-layered. 
At the same time more radial walls appear in the exomeristem 
(figs. 32, 88, 34). By the time this stage is reached, divisions 
make their appearance in the endomeristem in a growing point 
that is giving rise to the seta; although, if the sections are made 
through a young theca, it will be found that the endomeristem 
has not yet begun to divide any further. In the former case, at 
* Russow’s terms exomeristem and endomeristem have been adopted and 
applied to the meristems of the sporophyte of the Moss, because it seemed, on 
the one hand, that it would be a mistake to invent new ones; and, on the 
other, the tissues produced in the case of the Moss from its two meristems are 
homologous with the tissues produced from the two meristems in the — 
in those cases, at any rate, in which the vascular system 18 axial. Therefore 
have called the meristem which produces the central strand the endomeristem ; 
and the meristem which produces all the tissues outside ths central strand, 
namely the epidermis and cortex, the exomeristem. Russow's use of the terme 
in regard to the Lycopodiacem is that nearest to the use which I have here 
ventured to make. ; 
LINN. JOURN.— BOTANY, VOL. XXIV. Z 
Bassi ei. 
SE ee 
