PRASIOLA. 



[ 630 ] 



PREPARATION. 



suft, pellucid, longer, mostly empty, rarely 

 contaiuing- a persistent primordial utricle. 



Orthotrichace^. Basilar cells of the 

 leaves with only the reiy lowest soft, the 

 upper mostly thickened, rarely pellucid and 

 normal. 



PRASI'OLA, Meneg-hini.— A genus of 

 UlvaceiB (Confervoid Algre), separated from 

 Monostroma by the arrangement of the 

 quadri geminate cells of the frond in lines, 

 with wide intercellular walls ; from TJlva 

 by the existence of only a single layer of 

 cells, and from both by the absence(?) of 

 a reproduction by zoospores ; from Schizo- 

 yonium by the frond consisting of expanded 

 plates. The species are included imder 

 Ulva (the terrestrial forms) in the Br. Flora 

 and Harvey's AJgte, ed. 1. They have been 

 carefully examined by Jessen, who finds 

 the fronds proliferous at the margins ; the 

 " spores " he describes as consisting of mo- 

 tionless cells formed of the entire contents 

 of the cells of the frond, set free by the 

 solution of the parent cell. The reproduc- 

 tion of this group seems to us to require 

 further iuvestigation. Jessen includes here 

 P. calophylln (PI. 3. fig. 10), crispa,fur- 

 furacea, and a form which he names P. 

 stipifcita, differing from the last chiefly 

 in the narrowly wedge-shaped, stipitale 

 character of the frond : probably the last 

 three constitute only varieties of one species. 



BiBL. Jessen, Frasivlte Monoy. 1848 ; 

 Harvey, Br. Aly. 171 ; Hassall, Aly. 297, 

 pis. 77, 78 ; Kiitz. Sj). Aly. 472 ; Rabenht. 

 Ah/, iii. 308. 



PREPARATION of microscopic objects 

 for examination and preservation. — Some 

 remarks on the former point will be found 

 in the Introdxjction, p. xxxii ; and imder 

 many of the general and special articles, 

 such as Diatomacese, Ovule, &c., directions 

 are given. A few general remarks may 

 still however be made in this place. 



The parts of bodies or objects are usually 

 separated by the mounted needles under a 

 dissecting microscope, or by means of sec- 

 tions, according to the nature of the views 

 which it is desired to obtain. With regard 

 to the former operation, it need be merely 

 observed that it is generally to be per- 

 formed under water, in a watch-glass, glass 

 cell, or other convenient holder. 



The preparation of sections is a more 

 comyjlicated process. Soft parts of animals 

 are best sliced by means of a Valentin's 

 knife : but Armor 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 away. Fresh 

 stems, thick leaves, &c., may be simply 

 held in the fingers ; thin objects, such as 

 leaves, petals, &c., are best placed in a split 

 cork or piece of carrot, the halves of which 

 are kept together by insertion 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 softened by soak- 

 ing in water before slicing. Small firm ob- 

 jects, such as softer seeds, are mostly 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 sec- 

 tions of vegetable objects are obscured by 

 air-bubbles engaged in the intercellular pas- 

 sages, &c. In old wood and similar objects 

 immersed in liquid, the air is readily driven 

 out by heat ; in fresh structures, where 

 heat may coagulate or dissolve matters, the 

 air may be allowed to dissolve or escape by 

 itself, which requires time, or it may be 

 removed by exhaustion under an air-pump. 



Sections of wood, &c., which are to be 

 mounted in liquids, should be soaked for 

 some little time in spirit or turpentine, to 

 remove resins, &c. The Section-cutter is 

 used for slicing such objects ; but this is 

 not of much use except when large numbers 

 of very perfect sections of the same kind 

 are required for purposes of sale, &c. 



It need scarcely be said that sections 

 require to be made in various directions in 

 studying objects by these means. Thus 

 stems should be sliced horizontally, and 

 perpendicularly both parallel and at right 

 angles to the medullary rays. A\"hen work- 

 ing with high powers, it is necessary to be 

 on our guard against appearances of stria- 

 tion or fibres which maybe produced by the 

 fine notches in the cutting instrument. 



The structure of laminated shells, Szc, 

 may often be seen in fragments broken off 

 by the point of a knife. But sections of 

 bone, shell, kc, are best made by sawing 

 off thin pieces with a frame-saw having a 

 watch-spring blade, grinding them down 



