Deo. 26, 1884.] 



KNO\VLEDGE ♦ 



621 



nutrition and general self-interest, the different individuals 

 in the net like colony enter into a reproducuve stage, 

 whereby the preservation of the race will be secured after 

 th<3 death of the now-existing family. The changes about 

 to take place within the cell at this period are preceded by 

 a loss of the bright green colour so characteristic of the 

 organism, the starch also disappears, while a vast number 

 of little clear specks arise throughout the now quickly 

 modifying cell-contents. In a short time the protoplasmic 

 mass is broken up into a very great number (7,000 to 

 I'O.OOO) of roundish grains ((/onidia), which for a space of 

 half-an-hour or so move with a tremulous motion within 



OOo 





4. 



■■^OO'x) ^-or^fTOG T^T^O OOP 



—Portion of a cell of Water-net (magnified 400 times), 

 showing the formation of Micro-gonidia. (Cobn.) 



the cell (Fig. 4). When they come to rest, they arrange 

 themselves by force of heredity, after the \isual plan of 

 cell-grouping observable in the adult colony ; then when 

 the old wall of the mother cell is finally absorbed, or other- 

 wise destroyed, a brood of cohering sister Hydrodictyon 

 plantlets escape to lead an independent existence in the 

 surrounding water. The individuals rapidly elongate, and, 

 still continuing to remain attached by their ends, the 

 group soon assumes the appearance of a miniature water- 

 net. After this, cell enlargement proceeds most actively ; 

 and, at the end of three or four weeks, the colony will have 

 attained full size and maturity. 



©©J® 



Fig. 5. — Single ilacro-gonidia (magnified COO times). 



There is another, but less usual, way by which the 

 Hydrodictyon plant multiplies itself. Each individual 

 ceil, instead of breaking up its contents into the compara- 

 tively large gonidia (hence known as macro-yonidia), abov 

 ceferred to, resolves its protoplasmic mass into an enormou 



iig. G. — Portion of cell (magnified 30U limes), slio>>ing escaping 

 ilicro-ganidia. (Cohn.) 



number (30,000 to 100,000) of inconceivably small gonidia 

 {micro-r/onidia), which, instead of cohering amcng them- 

 selve?, rem*in perfectly free from one another and t. vcntnally 



escape (Fig. G), as unwalled and ciliate spores (Fig. 7), by 

 the rupture of the old mother wall. By the agency of their 

 cilia they are able to move hither and thither throughout 

 the water and mix with similar t-pores, set free from the 

 cells of other water-nets. It would seem that these motile 

 bodies (generally described as zoospores) are of sexual phy- 



^rt 



Fig. 7. — Single Micro-gonidia (magnified 600 times). 



siological importance, and tHat they conjugate with one 

 another during their brit-f period of activity. Toe con- 

 ditions under which the act of conjugation is effected are, 

 of course, entirely against the chances of self-fertilisation, 

 and thus the well-known benefits of a "cross" are fully 

 secured to the next succeeding algal generation. Imme- 

 diately upon conjugation beinij completed the dual mass 

 draws in its cilia, rounds itself off, forms for itself a resist- 

 ing cell-wall, and falls to the u.ud at the bottom of the 

 ditch, within or on the surface of which it lies in a still 

 or dormant condition fur a period of three or four months. 

 On the approach of spring vital activity is again resumed, 

 and for the next few months growth and development are 

 strictly confined to an enlai-gement of the cell. When 

 the cell has attained its maximum size, its contained 

 protoplasm breaks up into three, four, or five large 

 ovate zoospores that generally escape and move freely 

 through the surrounding water. But they soon become 

 motionless, and develop into single, mauy-sided cells, 

 from the angles of which long, horn-like processes in- 

 variably grow. Finally, the contents of each polyhedron 

 resolves itself into a group of two or three hundred gonidia 

 enclosed in an extremelj- delicate sac that protrudes ifc-elf 

 from the burst side of the containing cell, and these, after 

 about half-an-hour's tremulous motion, come to rest and 

 arrange themselves into a miniature net-like structure 

 which, eventually bursting the womb, escape, and, still 

 remaining attached to one another, gradually increase in 

 size and grow into a mature Hydrodictyon, only differing 

 from a noQ-sexually produced "net" in having a fewer 

 number of cells. But, such cells or individuals as these 

 that are the resulting offsprina; of a sexual act have had 

 their vital energies as it were renewed ; their constitutional 

 vigour and reproductive powers have been considerably 

 increased, and they have acquired a potential of growth 

 and multiplication that will enable them to run through 

 Eeveral non-sexual generations before they have exhausted 

 the present stock of available hereditary energy. 



Tabulating, then, by way of a summary, the entire 

 possible life-history of Hydrodictyon, or water-net, we 

 have : — • 



Non sexDal generation multiplying by the formation of macro- 

 gonidia repeated for many generations. 



Sexual generation multiplying by formation of micro-gonidia or 

 zoospores. 



Coiijugation. 



Resting spore. 



Renewal of growth and enlargement of cell. 



Conversion of entire cell-contents into 3 — 5 large zoospores. 



Development of these into " polvhedra." 



Formation of nnmerons gonidia within the polyhedra. 



Arrangement of gonidia into an ordinary net colo y. 



Non-sexual generation multiplying by maero-goni :a repeated for 

 many generations. 



We are asVed to state that Mr. Higgins's book, " Museums of 

 Natural History," which was reviewed on p. 4S8, is published by 

 D. Marples & Co., of Liverpool. 



