POTTIOIDEJE. 



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PREPARATION. 



posed of sixteen lanceolate, or subulate, entire 

 or cribrose equidistant teeth. 



POTTIOIDE^E. A family of operculate 

 Mosses belonging to the Acrocarpi, but 

 sometimes Pleurocarpous by innovating 

 branches. Leaves of very varied form, with 

 a terete nerve ; cells parenchymatous, per- 

 fectly hexagonal or squarish six-sided, always 

 looser at the base, sometimes very lax, more 

 or less pellucid, often exceedingly transpa- 

 rent, large, fragile, rigid, foraminate, bearing 

 on the upper side solitary papillae or several 

 confluent papillae (hence often truncate and 

 tuberculate at the apex), placed in the mid- 

 dle of the cell ; cells mostly full of chloro- 

 phyll, sometimes with a primordial utricle, 

 often very small and thickened. Capsule 

 erect, rarely inclined, oval, elliptic or pear- 

 shaped oblong, smooth or striate, the oper- 

 culum mostly conical or beaked. 



This family is divided into three tribes : 



1. CALYMPERACE^E. Basilar cells of 

 the leaves rigid, hyaline, often very brittle, 

 more or less ample, empty, distinctly fora- 

 minated. 



2. POTTIACEJE. Basilar cells of the leaves 

 soft, pellucid, longer, mostly empty, rarely 

 containing a persistent primordial utricle. 



3. ORTHOTRICHACE^E. Basilar cells of 

 the leaves with only the very lowest soft, 

 the upper mostly thickened, rarely pellucid 

 and normal. 



PRASIOLA, Meneghini. A genus of Ul- 

 vaceae (Confervoid Algae), separated from 

 Monostroma, Thuret, by the arrangement of 

 the quadrigeminate cells of the frond in lines, 

 with wide intercellular walls, from Ulva by 

 the existence of only a single layer of cells, 

 and from both by the absence (?) of a repro- 

 duction by zoospores; from Schizogonium 

 by the frond consisting of expanded plates. 

 The species are included under Ulva (the 

 terrestrial forms) in the Brit. Flora and 

 Harvey's Algce, ed. I. They have recently 

 been examined by Jessen, who finds the 

 fronds proliferous at the margins; the ' spores' 

 he describes as consisting of motionless cells 

 formed of the entire contents of cells of the 

 frond, set free by the solution of the parent- 

 cell. The reproduction of this group seems 

 to us to require further investigation. Jes- 

 sen includes here the British species, P. ca- 

 lophylla, crispa, furfuracea, and a form 

 which he names P. stipitata, differing from 

 the last chiefly in the narrowly wedge-shaped, 

 stipitate character of the frond; probably 

 the three last constitute only varieties of one 

 species. 



BIBL. Jessen, Prasiolte Monograph. Kiliae, 

 1848; Harvey, Brit. Alg. ed. 1. p. 171 ; 

 Hassall, Brit. Freshw. Alg. p. 297. pi. 77, 

 78; Kiitz. Sp.Alg.pM72. 



PREPARATION of microscopic objects 

 for examination and preservation. Some 

 remarks on the former point will be found 

 in the INTRODUCTION, p. xxviii. ; and under 

 many of the general articles, such asD i ATO M A- 

 CE.E, COAL, OVULE, See., special directions 

 are given. A few general remarks may be 

 offered in this place. The parts of bodies 

 are separated by means of the mounted 

 needles under a dissecting microscope, or by 

 means of sections, according to the nature 

 of the views which it is desired to obtain. 

 With regard to the former operation, it need 

 be observed merely that it is usually to be 

 performed under water, in a watch-glass, 

 glass cell, or other convenient holder. 



The preparation of sections is a more com- 

 plicated process. Soft parts of animals are 

 best sliced by means of a Valentin's knife ; 

 but firmer structures, such as horn, may be 

 cut with a sharp razor. Vegetable structures 

 in general are sliced with a razor, which 

 must be kept very sharp, and rubbed on a 

 strop frequently while in use, and always 

 before putting it away. Fresh stems, thick 

 leaves, &c. may be simply held in the fin- 

 gers; thin objects, such as leaves, petals, 

 &c., are best placed in a split cork, the 

 halves of which are kept together by inser- 

 tion in the neck of a vial or a test-tube, 

 which at the same time serves as a handle. 

 Sometimes it is advantageous to immerse 

 objects, especially soft or very small ones, in 

 thick mucilage of gum-arabic, and to allow 

 this to dry until tough enough to be cut by 

 the razor ; the slices are freed from gum by 

 immersion in water. Dry objects, such as 

 wood, dried leaves, seeds, &c., must be soft- 

 ened by soaking in water before slicing. 

 Small firm objects, such as seeds, are most 

 easily sliced when fixed in a bit of white 

 wax or stearine, which may be done by 

 placing them on the surface of the latter, 

 and stirring them into the substance melted 

 by the application of a hot wire. Most slices 

 of vegetable objects are obscured by air- 

 bubbles engaged in the intercellular pass- 

 ages, &c. In old wood and similar objects 

 the air is readily driven out by heat ; in fresh 

 structures, where heat may coagulate or dis- 

 solve matters, the air may be allowed to 

 dissolve or escape by itself, which requires 

 time, or may be removed by exhaustion. A 

 substitute for a regular air-pump may prove 



