REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



219 



brane.* When two fronds are about to con- 

 jugate, they place themselves parallel and op- 

 posite to each other, with their concave sur- 

 faces facing. We next remark that the cell 

 membrane partially gives way at the line above 

 mentioned, the two halves of each Clostcrium 

 separating slightly on the side opposite its 

 fellow, but remaining in contact on the other 

 sicle. The openings are soon observed to be 

 occupied by cushion-like projections of the in- 

 ternal membrane, which squeeze out between 

 the valves. From the fact that the cavity of the 

 internal membrane is double, or rather that each 

 halfof the Closterium possesses an independent 

 primordial membrane, it follows that each of 

 the projections above mentioned consists of 

 two distinct sacculi. Soon the two double 

 cushions come in contact ; they are at first 

 perfectly colourless, but shortly afterwards 

 become filled with green granular matter, and 

 press so closely together as to be no longer 

 distinguishable. It is next observed that from 

 the junction of the four sacculi, two canals 

 have resulted, each of which soon swells out 

 in a hemispherical form, corresponding to 



Fig. 136. 



Conjugation of Closterium. 



The two fronds are connected by two delicate tubes? 

 each of which contains a hemispherical germ- 

 cell closely invested by its membrane. The two 

 germ-cells, which are in opposition by their flat 

 surfaces, appear as one. About 40 diam. 



that of a mass of green granular matter which 

 now occupies its cavity. This mass is soon in- 

 vested by a delicate membrane, which, in the 

 progress of development, thickens and pre- 

 sents an uneven surface. The two bodies 

 which thus take their origin are the germ 

 cells. They soon become free from the struc- 

 ture in which they were formed, and, according 

 to Morren, display for about fifteen minutes 

 after their escape an active motion. After 

 this period, the motion ceases, and they attach 

 themselves to a foreign body. Morren has 

 observed their germination. The spherical 

 germ lengthens first at one, then at the oppo- 

 site extremity, so as to assume the charac- 

 teristic crescentic form of the plant. Its 

 green contents divide into two masses, eacli 

 of which is invested by a separate primordial 

 membrane, and occupies one of the future seg- 

 ments of the frond. In a short time the young 

 Closterium completely resembles the adult. 

 It is worthy of remark, that in the abnormal 

 cases in which only one germ results from 



* Morren, Ann. des Sc. Nat. 1 Ser. p. 325. 



the conjugation of two individuals, only one 

 of the halves of each empties itself, the other 

 remaining unaltered.* 



In other families of Desmideas, the pro- 

 cess of conjugation, although variously modi- 

 fied as to its less important details, is essentially 

 the same as that which occurs in Closterium.f 



17. In the Zygnemacese, confervoid plants, 

 which seem to have a close relation with the 

 Desmidese, the phenomena of conjugation have 

 been long known. The frond consists of a 

 series of cylindrical cells, which lengthens 

 indefinitely by repeated division of its ele- 

 ments. Here, as in the Desmideae, it is the 

 last-produced cells in the filament which take 

 part in the process of conjugation. In 

 Spirogyra the union of two cells belonging 

 to opposite filaments takes place by the ex- 

 pansion of one side of each, so as to form a 

 papilla, or short tube with a rounded end. The 

 ends of the two projections then come into 

 contact, become slightly flattened as they are 

 pressed against each other, and unite. The 

 double wall formed by their union, dissolves, or 

 is broken through, so that a free passage is es- 

 tablished between the two cell cavities. Upon 

 this, the whole of the chlorophylle previously 

 arranged round the inside of each of the cells, 

 becomes a confused mass, which soon forms 

 itself either in the cavity of one of them, or 

 in the connecting canal, into a globular or 

 oval smooth spore, invested with a colourless 

 cellulose membrane. Having arrived at this 

 condition, it remains several months from 

 the autumn of one year to the spring of the 

 following, without undergoing any change of 

 form. | During this period two new membranes 

 are produced within the first by the secretion of 

 cellulose on the surface of the primordial utri- 

 cle. Of these two, the external is of consider- 

 able thickness, and of a yellow colour. The 

 internal, which may be considered as the 

 proper membrane of the spore, is delicate and 



* According to Morren, the process above de- 

 scribed is not the only one by which the reproduc- 

 tion of Closterium takes place. In the green granular 

 matter contained in a frond, there occur spherical 

 corpuscles which, according to that observer, are 

 capable of reproducing the parent plant. He has 

 described and figured their germination, and it is 

 worthy of remark that his figures of the earliest 

 stages ofClosteriathus developed, correspond closely 

 with those of the earliest stages of the plant as ob- 

 served by Mr. Ralfs, who, however, assigns to them 

 a different origin. (See British Desmidece, tab. 

 xxvii. in.) 



f It is clear that if the formation of germs by 

 conjugation were the only provision for the repro- 

 duction of the species, in the Closteria and many 

 other families of DesmideaB, its total disappearance 

 must result, inasmuch as the conjugation and con- 

 sequent destruction of a pair of Closteria can only 

 give rise to an equal orless number of new individuals. 

 But the other mode of reproduction already alluded 

 to as occurring in Closterium, and which has been 

 so well described by Mr. Ralfs in the other Dos- 

 midea?, affords an effectual safeguard against their 

 otherwise possible extinction. 



J Braun has observed the germination of the 

 spores in a specimen of Spyrogyra setitbrmis which 

 had been collected for eleven months. (Braun, 

 1. c. p. 144.) 



