256 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
As development proceeds, the dorsal portion of the disk increases 
further in size and develops active photosynthetic tissue with air 
spaces in several layers. Those of the uppermost layer are bounded 
on the outside by an epidermis with pores of the compound or dolio- 
form type. Sometimes the surface is fairly smooth, but in certain 
species the ceiling of each chamber projects in a vaultlike way, the 
surface thus becoming coarsely tuberculate. In A. echinella the pro- 
jections reach an extreme development, attaining a height of a milli- 
meter, and the surface acquires an almost spiny appearance. Some- 
times the margin of the disk remains undivided, but more or less dis- 
tinct lobes are usually developed, each lobe corresponding to an 
archegonium or group of archegonia. On the ventral surface two 
protective structures, in addition to the calyptra, make their appear- 
ance, although they remain abortive in the absence of fertilization. 
These structures are the involucre and the pseudoperianth. 
The involucre, as Leitgeb noted, is not uniform throughout the 
genus. In the more usual cases it represents an outgrowth of the 
sides of the groove in which an archegonium (or group of archegonia) 
is situated and is then continuous with the edges of the lobes, forming 
a thin membranous expansion. This expansion, which never incloses 
the sporophyte on the outer side, is only one cell thick along the mar- 
gin and is composed of colorless cells, among which scattered cells 
with oil bodies stand out conspicuously. When the margins of the 
lobes are strongly involute, the involucre is relatively narrow, as in 
A, palmeri, but this condition is exceptional, the involucre often at- 
taining a width of 1 to 1.5 mm. Between the sporophyte and the 
peduncle the halves of the involucre become continuous, although 
there is sometimes a deep indentation in this region. When it is un- 
usually deep, as in A, californica, the involucre appears to be divided 
into two parts. The involucre in such species as A. africana deviates 
somewhat from this account, as Leitgeb clearly shows. It repre- 
sents a short membranous flap between the sporophyte and the 
peduncle, gradually narrowing out on the sides and reaching only 
part way to the margin. In all cases the margin of the involucre is 
either entire or vaguely and irregularly crenulate or denticulate. 
The pseudoperianth is the most distinctive structure found in the 
genus and at once separates As¢erella from all the other genera of 
the Operculatae. It consists of a tubular membranous sheath, nar- 
rowed at the apex to a small pore but more or less strongly inflated 
throughout the rest of its extent. When the sheath is young it is 
perfectly continuous, and this condition seems to be long maintained 
in the Persian Fimbriaria silachorensis Schiffn.,) a species allied to 
A. ludwigit. In most cases, however, the pseudoperianth becomes 
* Oesterr. Bot. Zeitschr. 58: 229. pl. 7, f. 1. 1908. 
