REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



tions. In the history of their development 

 we have an epitome of that of all the more 

 simple Algae. In the family of Protococci 

 the type may be said to be included to which 

 all the green and olive-coloured Algae, with 

 the exception, on the one hand, of the.Des- 

 midese and their allies, on the other of the 

 Fucaceae, may be referred. Among the Des- 

 mideae, indeed, is placed one genus, that of 

 Pediastrum, in which the zoosporous is the 

 only form of reproduction which has been 

 observed. Pediastrum consists of a disc of 

 cells, which are usually eight in number, and 

 contain a protoplasma, which possesses a 

 generally diffused green colour. The first 

 step in the reproductive process consists in 

 the separation of the protoplasma into a green 

 and a colourless portion. The former, after 

 collecting- mto a central mass, becomes divided 

 into numerous secondary masses, the number 

 of which is always a power of two. From the 

 latter is formed a transparent gelatine-like in- 

 vestment which lines the parent cell. After the 

 completion of these changes the original cell- 

 wall is ruptured, and the whole contents 

 escape in a mass. No sooner has this taken 

 place than the corpuscles into which the green 

 protoplasma has divided, commence an active 

 motion in the interior of the gelatinous cell 

 in which they are included, and in fact display 

 in every respect the form and peculiarities of 

 zoospores. They are not, however, as in 

 every other example with which we are ac- 

 quainted, destined to display their activity 

 beyond the narrow limits within which they 

 originate. In a short time their motions be- 

 come languid, and finally cease, while they 

 arrange themselves in a beautifully regular 

 geometrical order which corresponds exactly 

 to that of the cells that constitute the adult 

 Pediastrum. The next change observed is 

 the disappearance of the gelatinous membrane, 

 and the investment of each of the zoospores 

 with a distinct covering of its own. From this 

 there results a disc-like body, which, in a 

 short time, assumes all the characteristics of 

 the original parent.* 



3. Taking these simplest of unicellular 

 plants as our point of departure, we pass to the 

 consideration of the confervoid Algae, many 

 of which, though they are but little elevated 

 above the Protococci as regards their struc- 

 tural elements, present a general appearance 

 which at first sight recalls that of plants very 

 much higher in the scale of organisation. 

 Thus in Bryopsis and its allies, in which the 

 tubular frond branches in the most compli- 

 cated manner, the whole consists essentially 

 but of a single cell, the cavity of which is 

 continuous throughout. When the formation 

 of zoospores is about to take place, all that 

 is observed in a Bryopsis is the accumulation 

 of the green granular protoplasma towards the 



* The development of Pediastrum has been de- 

 scribed by Braun (Die Verjungung in der Natur) as 

 well as by Caspary (Botanische Zeitung, 1850, 

 S. 786.). The description in the text is after Braun, 

 with whom Caspary agrees in every important par- 

 ticular. 



213 



extremities of the tubular branches. In these 

 situations the cavity of the tube becomes 

 completely filled, while at some point in the 

 neighbourhood of each accumulation, the tube 

 membrane becomes sacculated so as to present 

 a nipple-shaped projection. In the meantime 

 the accumulated protoplasma is observed to 

 have given rise, by its division, to numerous 

 green bodies, the forms of which cannot yet 

 be distinguished, owing to the closeness with 

 which they are packed together. No sooner, 

 however, is this process complete, than a re- 

 markable phenomenon, corresponding to that 

 already described in Protococcus, manifests 

 itself. The crowded zoospores, now com- 

 pletely developed, at once commence their 

 characteristic motions. From this results an 

 appearance of confused agitation, to which 

 the term " swarming " has been applied by the 

 Germans. A minute aperture, or pore, is 



Fig. 129. 



a, termination of tubular frond of Bryopsis crowded 

 with zoospores ; b, the same, after the escape of 

 its contents. Each of these exhibits the lateral 

 pore ; c, fully formed zoospores in active motion, 

 a and 6 150 diam, c 200 diain. 



then found at the extremity of the nipple-like 

 projection, apparently in consequence of the 

 absorption of the cell-membrane at its apex. 

 The zoospores now begin to escape, at first 

 one by one, afterwards more rapidly, until at 

 last a few only are left occupying the cavity of 

 the tube.* 



4. In the simplest forms of jointed con fer- 

 vokls, the frond consists of a series of cells super- 

 posed one upon the other, each of which is 

 capable of producing zoospores independently 

 of the rest. In the vegetative state, each 

 contains only a green protoplasma. The re- 

 productive process is the same in every 

 respect as in the Bryopsideae, the opening by 

 which the zoospores make their exit, being 

 situated at the upper part of the cell, imme- 

 diately below the septum, which divides it from 

 its successor. In other cases (as in Mi- 

 crospora), the zoospores escape by a kind of 



* Thuret, Recherches sur les Zoospores des Al- 

 gues. Ann. des Sc. Nat. xiv. 217. 



p 3 



