RER 
tudinal axis of the frond, thus establishing a longitudinal connection between the cells 
of the flat. A longitudinal arrangement of the cells not agreeing with the distribution 
corresponding to the succession of the cell-divisions then frequently appears (fig. 449). 
Adventitious shoots arise in various number from the axils of the branches; 
they have no fixed position but develop from marginal initial cells or from other 
cells near the margin (fig. 450). 
In the Baltic Sea a very narrow form occurs, the frond 
L, 
N! of which is only c. 1 mm broad, at Bornholm even only 
| c. 0,3 mm broad (Plate VII fig. 7). While in the typical 
Sh form the frond is rounded above with two or three 
N) branches near the top, the frond from the Baltic is much 
| 
less branched; the shoots are pointed, frequently without 
any branch near the top (fig. 447). The frond is poly- 
stromatic almost to the margin. Of the segments of the 
second order only one or two are included; sometimes 
even the first one reaches the margin of the frond (fig. 447, 
7th segment from the top). 
The mid-rib becomes very convex on both sides. 
Within a thin assimilatory cortex it is built up by 1) 
primary longitudinal cell-rows, the cells of which are 
prineipally jointed by secondary pits, often three in each 
transversal wall, 2) conducting hyphæ given off from the 
former (comp. WILLE 1887 p. 62 figs. 14—20, 79). In a 
cross section a transversal row of primary cells is present, 
the middlemost of which, the primary axial cell, may have 
a smaller diameter than the others. All the inner cells of 
the mid-rib contain several nuclei. 
St As, Sporelings produced in cultures were not obtained 
Membranoptera alata. Young plants And very young plantlets were not met with in Nature, 
growing on Membranipora pilosa. but young plants that could be referred with certainty to 
een this species were collected in July, e. g. growing on Mem- 
near the base, 19:1. branipora pilosa at Herthas Flak. They were !/; to !/, cm 
long, not branched, with a terete stipe continuing in a 
vigorous mid-rib from which opposed lateral veins were given off (fig. 451). 
The apical cell-divisions agreed exactly with those of the adult plants (fig. 452). 
In one of the larger plants the ramification had begun, the first branch hav- 
ing been formed near the top (fig. 451 A). As shown. in fig. 452, this branch 
differs from those of the older fronds in that it is more symmetrical, the first 
segments being equally developed in the anterior and the posterior side; the foll- 
owing segment of the primary axis is therefore included, and the rib of the branch 
is not as usual in its outer part situated in the anterior border of the segment of 
the mother axis from which it has arisen. The primary axis appears to develop 
LL LL LU 
