PRITCHARDIA. 



[ 638 ] 



PROSTHEMIUM. 



cell presents a complex arrangement in 

 some cases : Braun correctly distinguishes 

 three layers in Hydrodictyon ; there are 

 three in Chara, where the intermediate one 

 contains the chlorophyll-granules, and the 

 innermost forms the circulating mass; a 

 distinct layer is left after the discharge of 

 the zooapores in Cladophora, &c. Thus, as 

 explained under the head of CELL-forina- 

 tion, the primordial utricle or formative 

 protoplasmic layer is the active agent in 

 cell-division, and the layer forming the 

 surface of the isolated portions of contents 

 of parent cells produces the new cell-wall 

 in all cases of free-cell formation, whether 

 taking place in parent cells, or, as in the 

 case of the zoospores of Algae, after escape 

 from the latter. Strassburger describes 

 the primordial utricle as being radiately 

 striated, especially when treated with osmic 

 acid. 



In many of the Algae, some of the indivi- 

 dual cells regularly exist for a certain period 

 as masses of protoplasm devoid of a cellulose 

 coat, as, for example, the spores of Fucus \ 

 and its allies, and the active zoospores of 

 Conferyoids; and these bodies, although 

 presenting a well-defined outline, do not 

 appear to have a properly developed mem- 

 brane on the surface, which merely appears 

 to be denser than the semifluid central por- 

 tion. These bodies withdraw themselves 

 evidently from the definition of a vegetable 

 cell as ordinarily given; nevertheless they 

 constitute all the essential living part of a 

 vegetable cell, and indicate most clearly 

 the undoubted fact that the cellulose walls, 

 that is to say all the really solid and per- 

 manent portions of vegetable structure, are 

 mere skeleton or shell for the protoplasmic 

 or nitrogenous structures. 



BIBL. Mohl, Bot.Zeit.\\. 273; Vcrmischte 

 Schrift. 362; Henfrey, Ann. N. H. xviii. 

 364; Nageli, Zeitschr. wiss. Sot. 1844 & 

 1846; Braun, Verj'iing., Hay Soc. 1853, 

 121 ; Cohn, Nova Acta, xxii. 605 ; Prings- 

 heim, Bild. d. Pftanzenzelle, 1854 ; Hartig, 

 Bot. Zeit. xiii. 393 ; Criiger, ibid. 601 ; Sachs, 

 Bot. 43 ; Henfrey-Masters, Bot. 495. 



PRITCHAR'DIA, Raben A genus of 

 Diatomaceae, comprising certain species of 

 Nitzschia and Synedra. (Rabenht. Alg. i. 

 162.) 



PROBOSCEL'LA, Kt. A genus of 

 Holotrichous Infusoria. Elongate-clavate, 

 compressed, with aii apical hook; mouth 

 ventral ; an undulating membrane between 

 the front and mouth; with one or more 



caudal setae. P. vermina ; marine length 

 1-250". 



PROEMBRYO. The term applied to 

 the structure first produced from the ger- 

 minal vesicle of Flowering Plants, after im- 

 pregnation, consisting of the suspensor and 

 the embryonal cell at its extremity. The 

 proembryos of the Gymnosperms are espe- 

 cially remarkable (see OVULE). The same 

 term is often incorrectly applied to the pno- 

 THALLIUM, the cellular structure first pro- 

 duced in the germination of the spores of 

 the higher Flowerless Plants. 



PROROCEN'TRUM, Ehr. A genus of 

 Cilio-flagellate Infusoria. 



Char. No eye-spot ; carapace smooth, 

 terminating in a point in front; flagellum 

 single. 



P. micans (PI. 31. figs. 70 & 71), ovate, 

 greatly compressed ; length 1-430". 



BIBL. Ehr. Inf. 44 ; Clap, et Lach. Infus. 

 411 ; Kent, Inf. 461. 



PROR'ODON, Ehr. A genus of Holo- 

 trichous Infusoria, family Trachelina. 



Char. Body covered with vibratile cilia, 

 truncate in front ; mouth with a cylinder of 

 teeth ; freshwater. Species numerous. 



P. teres (PI. 31. fig. 72). Body ovate, 

 terete, white. Length 1-140". 



BIBL. Ehr. Inf. 315; Clap, et Lach. 

 Infus. 318 ; Kent, Inf. 491. 



PROSEN'CHYMA. See TISSUES, Ve- 

 getable. 



PROSTHE'MIUM, Kunze. A genus 

 of Melanconiei (Coniomycetous Fungi), 

 growing upon the branches of trees, form- 

 ing circular depressed spots ; the perithecia 



Fig, 609. 



Prostheminm betnlinum. 



Spores and paraphyses seen in a vertical section of frut. 

 Magnified 200 diams. 



enclose erect articulated filaments bearing 

 radiating tufts of two or three septate spores 

 (fig. 609). P. hetulinum occurs upon the 

 bark of the branches of the birch tree. The 

 species are mere forms of Sphceriacei. 



