S14 



REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



dislocation of the tube, each cell dividing into 

 two in a plane parallel to the septa.* 



5. The Ulvaceae, among which the frond has 

 no longer the form of a filament, but assumes 

 that of a membranous expansion of juxtaposed 

 cells, still present the same phenomena to our 

 notice. In the cells set apart for the forma- 

 tion of zoospores, the green protoplasma is 

 increased in quantity, at the same time that it 

 becomes accumulated towards one point of 

 the cell-wall. As the zoospores are formed, 

 they are observed to converge with their 

 apices towards this point. The phenomena 

 attending their escape from the parent cell are 

 similar to those which we have already 

 noticed. 



6. In some genera, which seem to he closely 

 related in form and structure to the Bryop- 

 sideas, we observe this important difference, 

 that the zoospores are developed in an organ 

 specially destined to this purpose, which 

 presents peculiarities of form distinguishing 

 it from every other part of the branching 

 tubular frond. Thus in the genus Derbesia 

 distinct spore cases are to be observed, the 

 cavity of which does not communicate with 

 that of the frond. These organs, which are 

 of an oval form, take their origin in the same 

 manner as the ordinary vegetative branches 

 of which they are modifications. A young 

 branch which is destined to become a spore 

 case, instead of elongating indefinitely, begins, 

 after having arrived at a certain length, to 

 swell out into an ovoid vesicle, in the cavity 

 of which a rapid accumulation of protoplasma 

 takes place. The next change which oc- 

 curs is the separation of this protoplasma 

 from that of the rest of the plant with which 

 it was before continuous, so as to give rise to 

 an oval and opaque mass, which soon becomes 

 surrounded by a distinct membrane. As the 

 result of the division of this mass, a number 

 of pyriform zoospores, each of which is fur- 

 nished with a crown of cilia, are set free. 

 Many other genera have been described by 

 Derbes and Solierf , in which the relations of 

 the spore cases to the frond are similar to 

 those which exist in Derbesia, although the 

 forms presented by the organs in question are 

 infinitely various. 



7. The researches of the authors above 

 alluded to, along with those of M. Thuret, have 

 shown us that in many families of the olive- 

 coloured Algae, the occurrence of zoosporous 

 reproduction is no less general. The zo- 

 ospores, however, although they resemble in 

 their general form those of the plants which 

 we have been considering, differ from them 

 not only in respect of their olive colour, but 



* In the genus (Edogonium, the protoplasma of 

 each joint, instead of being converted into a number 

 of zoospores, goes to form but one, which differs 

 from those of other genera, in the first place in 

 being considerably larger, and secondly, in present- 

 ing around its rostrum not two, but a number of 

 cilia, which are arranged in the form of a crown. 

 (See Thuret, /. c. p. 226.) 



t Derbes and Solier, Sur les Organes reproducteurs 

 des Algues. Ann. des Sc. Nat. xiv. 260. 



in the arrangement of their cilia. These 

 organs, which are always two in number, are 



130. 





Sporangium of Ectocarpus siliquosus, 240 diam. 



Ectocarpus is one of the simplest forms of olive- 

 coloured Algae, consisting of branching, conferva- 

 like filaments. The extremity of any of the 

 branches is capable of being converted into a 

 sporangium by the absorption of the septa of the 

 terminal cells. The zoospores are arranged in 

 regular horizontal layers, the positions of which 

 are indicated in the empty sporangium by faint 

 markings of its membrane. 



usually of unequal length, and emanate not 

 from the beak, but from the reddish-coloured 

 point in its neighbourhood. The longest is 

 directed forwards, being closely applied to the 

 colourless beak ; while the other, which seems 

 during the motions of the spore to serve as a 

 rudder, assumes an opposite direction. In 

 many genera a peculiarity exists, the significa- 

 tion of which is not yet completely Bunder- 

 stood that, namely, of a double fructification. 

 The ovoidal sporangia (oosporangia, Thur.), 

 which have been frequently described as single 

 spores, in reality contain numerous zoospores. 

 The other form (tric/to-sporangium,Thur.) con- 

 sists of a series of small cells joined together 

 so as to form a narrow and generally short fila- 

 ment. Each of the cells contains a zoospore, 

 which, according to the observations of Thuret, 

 is no less capable of germinating than the 

 one produced by the oosporangium. In the 

 genus Cutleria there is observed, for the first 

 time, another feature of great interest and 

 importance ; namely, the appearance of two 

 kinds of organs which seem to be opposed to 

 each other as regards their reproductive func- 

 tions. The sporangia (trichosporangia) of 

 Cutleria, not only differ from those of other 

 genera, in respect of their greater size, but 



