ISOETES. 



[ 444 ] 



IXOPES. 



The contents of the microspores are at 

 first merely granular protoplasm. About a 

 month after they are scattered from the 

 sporange, the protoplasm of the cell becomes 

 divided into two or four portions, which 

 form cells, in each of which again are de- 

 veloped two vesicles, each producing a fila- 

 ment coiled up spirally. The spores swell, 

 the daughter cells burst, and the lenticular 

 vesicles escape ; the latter then open and 

 emit the spiral filaments, which are found 

 to be covered with cilia on the anterior 

 turns of the spiral, by means of which they 

 move actively through the water. They are 

 the spermatozoids. 



The macrospores when they escape from 

 the sporange, contain only protoplasm with 

 oil-globules. In the course of a few weeks, 

 the internal cavity of the spore begins to 

 exhibit a development of cellular tissue, by 

 which it is subsequently filled up ; this is 

 the pro/thallium. At the same time, the in- 

 ternal coat increases in thickness, and ex- 

 hibits several layers. The increase in size 

 of the prothalliimi causes the spore-coat to 

 burst at the apex where the three ridges 

 meet, so that three triangular valves turn 

 back, exposing the prothallium. On this 

 are developed the archegonia, the first on 

 the apex in the central point where the 

 three points of the spore-coat meet. If this 

 is not fertilized, others are produced around 

 it. The archeyonium is of much the same 

 character essentially, as that of the rest of 

 the higher Cryptogamous Plants, consisting 

 of a papilla with a central canal leading to 

 the embryo-sac. The four rows of cells 

 forming the neck of the archegone separate, 

 and a germ-cell is formed in the embryo- 

 sac. This is fertilized by the entrance of a 

 spermatozoid into the embryo-sac. 



In the development of the embryo in the 

 spore, it forms a cellular body, which gra- 

 dually displaces the cellular tissue originally 

 filling this up. The first leaf and roots are 

 developed while the rudiment is still within 

 the spore-coat, in opposite directions, and 

 horizontally (right and left) in relation to 

 the apex of the spore. The young plant 

 somewhat resembles a germinating Mono- 

 cotyledon. 



The woody structure of the stem of Isoetes 

 consists of spiral-fibrous cells, usually annular 

 or retipuluted, but sometimes really spiral. 



Carruthers has explained the resemblance 

 of the method of the growth of Isoetes and 

 that of the gigantic Lepidodendron of the 

 Carboniferous deposits. 



BIBL. Bischoff, Crypt. Gewdch. Rhizoc., 

 Nuremberg, 1828,70; Mohl, Verm. Schrift., 

 Tubingen, 1845, 122 ; Miiller, Sot. Zeit. vi. 

 297, 1848 (Ann. N. H. 2 ser. ii. 81); 

 Mettenius, Btitr. z. Bot. Heidelberg, 1850 j 

 Hofmeister,^tM. sacks. Ges. d. Wiss.iv. 123 ; 

 Braun, Flora, 1847, 33; Carruthers, Lect. 

 Roy. Inst. 1869 ; Henfrey-Masters, J?(^. 1878. 



ISOT'RICHA, Hein. A genus of Holo- 

 trichous Infusoria =Opalince with a ventral 

 mouth. In the rumen of ruminants. 

 (Kent, Inf. 497.) 



ISTH'MIA, Ag. A genus of Diato- 

 maceae, fam. Biddulphiaceae. 



Char. Frustules depressed or subcylindri- 

 cal, rhomboidal or trapezoidal in front view, 

 angles more or less produced ; frustules co- 

 herent by the angles, basal frustule stipitate ; 

 surface of valves and hoop appearing reticu- 

 lar or cellular. Marine. Two British species. 



The depressions upon the valves and 

 hoop are so large as to produce a distinct 

 reticular or cellular appearance when viewed 

 by ordinary illumination. 



/. obliquata (nervosa, K.). V. with linear 

 thickenings, giving them a coarsely reticular 

 or veined appearance. 



I.enervis (PI. 17. fig. 2). Valves uniformly 

 covered with depressions. 



BIBL. Ehr. Inf. 209 ; Kiitz. Bacitt. 137, 

 and Sp. Alg. 135 ; Ralfs, Ann. N. H. 1843, 

 xii. 270 ; Rabenh. Alg. i. 309. 



ITCH-INSECT. See SARCOPTES. 



IVORY. This substance, which consists 

 of the tusks of the elephant, possesses the 

 minute structure of the ivory of teeth. 



IVORY, VEGETABLE. This substance, 

 consisting of the ALBUMEN of the seeds of 

 a Monocotyledonous tree, Phytelephas ma- 

 crocarpa, is composed of cellular tissue, with 

 the walls so thickened by horny secondary 

 deposits that the cavities of the cells are 

 almost obliterated. The pores of the se- 

 condary deposits, however, remain unco- 

 vered throughout all the thickening, and 

 thus are converted into tubes or canals run- 

 ning to meet each other from the small re- 

 maining cavities of contiguous cells. In 

 PI. 47, fig. 23 b represents a section mounted 

 in Canada balsam, which has in part pene- 

 trated into the cavities ; the remaining ca- 

 vities and pore-canals are filled with air and 

 thus appear black (a). 



IXO'DEA. A family of Arachnida, of 

 the order Acarina. 



Contains the genera IXODES and ARGAS. 



IXO'DES, Latr. A genus of Arachnida, 

 of the order Acarina, and family Ixodea. 



