PILOTRICHUM. 



[ 506 ] 



PILULARIA. 



septum; thus, in its simplest form, this plant 

 consists of only three cells ; subsequently it 

 becomes complex by the root-cell or myce- 

 lium producing numerous stolons. P. cry- 

 stallinus is yellowish at first, the peridiole 

 finally black. P. roridus, Bolt., a doubtful 

 form, is smaller and more slender than the 

 last, having an elongated filiform stem. 



BIBL. Berk. Brit. Fl. ii. pt. 2. p. 231 ; 

 Fries, Summa Veg. p. 487 ; Cohn, Nova 

 Acta, xxiii. p. 492. 



PILOTRICHUM, Pal. de Beauv. A 

 genus of Hypnoid Mosses, including some 

 Fontinales of authors. 



1. Pilotrichum antipyreticum, C. Miill.= 

 Fontinalis antipyretica, L. 



2. P. squamosum, C. Miill,=jP. squamosa, 

 L. 



3. P. ciliatum, C. Mull. = Ancectangium 

 ciliatum, Brid., var.y. striatum= A. ciliatum, 

 Wilson. 



4. P. heteromallum, P. B.=Daltonia he- 

 teromalla, H. and T. 



PILULARIA, L. A genus of Marsile- 

 aceous Plants, containing the only British 

 representative of the order P. globulifera 

 (fig. 579). This is an inconspicuous plant 

 growing in mud at the edges of or in pools, 

 having a filiform creeping stem, bearing 

 erect filiform green leaves and delicate adven- 

 titious roots, and producing shortly-stalked 

 globular spore-fruits, about the size of a 

 peppercorn. The anatomical structure of 

 the stem and leaves is simple; they are 

 clothed with an epidermis possessing sto- 

 mates, and a cross-section both of the stem and 

 the leaves exhibits a central vascular bundle 

 (of spiral vessels), surrounded by a sheath 

 of brownish cells, while in the delicate cel- 

 lular tissue intervening between the central 

 bundle and the epidermis stands a circle of 

 air-passages, separated from each other by 

 simple radiating cellular septa. 



Fig. 577. 



Fig. 578. 





Pilularia globulifera. 



Fig. 577. A vertical section of a spore-fruit. Magni- 

 fied 5 diameters. 



Fig. 578. Transverse section of a spore-fruit. Magni- 

 fied 5 diameters. 



The spore-fruits are hollow cases with an 



outer tough cellular coat, and an inner more 

 delicate coat dipping in at four perpendicular 

 lines, as far as the centre, so as to form dis- 

 sepiments dividing the first into four 

 chambers (figs. 5/7, 578) ; up the centre of 

 the outer wall of each chamber runs a raised 

 ridge (a kind of placenta), whence arise the 

 sporanges or thecce (fig. 5/8). These are 

 pear-shaped sacs composed of a very delicate 

 cellular membrane. Those in the upper 

 part of each chamber contain a number of 

 minute globular bodies, resembling pollen- 

 grains, immersed in a gelatinous liquid. The 

 sacs in the lower part of the chamber contain 

 only one body or spore, but this of very 

 peculiar form; it nearly fills the theca, is 

 somewhat oval in form, and possesses several 

 coats. 



The development of the spores, as de- 

 scribed by Valentine, is very curious ; the 

 small spores are developed in the usual way, 

 by the formation of parent-cells in the 

 theca, which parent- cells subsequently each 

 produce four spores. In the thecae which 

 have the single large spore, a number of 

 parent-cells are originally produced, and these 

 become divided into four chambers by septa, 

 but then all but one of these decay. This 

 produces four spores, but out of these four 

 only one attains to perfect development, the 

 rest being subsequently dissolved and ab- 

 sorbed to make room for the solitary large 

 spore. This reminds us in some degree of 

 the numerous germs formed in the Gymno- 

 sperms (OVULE), and subsequently absorbed. 

 The two kinds of spore in Pilularia corre- 

 spond to the two forms in S EL A GIN ELL A 

 and ISOETES, and to the pollen arid ovules 

 of the Flowering Plants. They are set free 

 by the dehiscence of the spore-fruit, and lie 

 at first imbedded in the jelly poured out by 

 the thecae. 



In this state the small spores exactly 

 resemble pollen-grains, having an outer 

 granular, and an inner delicately membra- 

 nous coat ; the outer coat presenting ridges 

 corresponding to the points of contact in the 

 parent-cell. When set free, the spores soon 

 burst at these ridges, and the inner coat is 

 slightly protruded; this next bursts and dis- 

 charges a number of lenticular cellules, from 

 each of which escapes a ciliated spiral sper- 

 matozoid. 



The mature large spores (fig. 580) are of 

 oval form, and have a thick, outer gelati- 

 nous coat composed of prismatic cells stand- 

 ing perpendicularly on an inner glassy coat; 

 the gelatinous coat is perforated at the 



