220 



REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



transparent. Germination consists in the 

 growing out of this membrane at one end of 

 the spore into a many-celled filament, which 

 escapes through a lacerated opening in the ex- 

 ternal membranes, and gradually assumes the 

 character, and appearance of the parent plant. 

 At the same time a tubular elongation of the 

 same membrane of limited growth is formed 

 in the opposite direction, which is the rudi- 

 ment of a root.* 



18. In a species of Palmelleae (Palmoglea 

 macrococca) in which the whole individual 

 consists of a single ovoid cell containing green 

 granular matter, and usually multiplying itself 

 by successive division, the phenomena of con- 

 jugation present themselves in a somewhat 

 different and very remarkable form. Here 

 two cells, probably the result of a series of 

 divisions, undergo a complete union, affecting 

 not only their contents, but also their mem- 

 branes. They coalesce as completely at their 

 points of contact, as two contiguous drops of 

 water, the result of their union being a cell 



which differs in no"respect from its predeces- 

 sors, except in the greater thickness of its 

 walls, and in the complete conversion of the 

 chlorophylle of its contents into oily globules. 

 Like the spore of the Zygnemaceae, it is des- 

 tined to a long period of inactivity, after 

 which, by the successive division of its con- 

 tents, it gives rise to a new series of individuals, 

 similar to those that preceded it. 



1 9. We have still to consider the most remark- 

 able condition under which conjugation takes 

 place among the Algae. The evolution of the 

 aggregate zoospore of Vaucheria has been 

 already described. In the plant which results 

 from its germination, Karsten has observed 

 that along the course of those filaments which 

 come in contact with the atmosphere are 

 formed organs of a peculiar structure. They 

 originate like the ordinary branches, as nipple- 

 shaped buddings out of the cell-wall, which 

 are distributed in pairs along the whole course 

 of the older filaments. In every pair of 

 organs, one elongates so as to form a closed 



Fig. 137. 



A. portion of the tubular frond of Conferva fontinalis, showing the arrangement of the sexual repro- 

 ductive organs. About 30 diam. (Karsten.) 



tube, which curves round into a spiral form, 

 like the leaves of a Pilularia, while its fellow 

 soon ceases to grow in length, but swells out 

 into a globular or oval form, about three times 

 as wide as the other. At first both contain 

 chlorophylle, which, in the tubular organ, is 

 soon replaced by colourless globules. In the 

 meantime its fellow, which resembles a dark- 

 green-coloured globe, supported on a short 

 pedicle, alters in form, its cell-wall extending 

 into a nipple-shaped projection on the side 

 next the tubular organ, with which it finally 

 comes in contact. This condition lasts for 

 some time, but it does not appear determined 



Fig. 138. 



A single group more highly magnified, about 200 diam- 

 Two of the egg-shaped organs which contain the 

 germs are represented; one of which is in con- 

 tact, by its smaller end, with the tubular organ 

 which occupies the centre. (Karsten.) 



with sufficient distinctness by Karsten's obser- 

 vations, that an actual interchange of the 



* Vaucher, Conferves d'Eau douce, p. 46 ; Pring- 

 sheim, Annals of Nat. Hist. June, 1853. (Trans, by 

 Mr. Henfrey.) 



contents takes place. All that we learn as 

 certain is, that after the completion of what 

 he calls the act of fructification, a newly 

 formed cell appears in the cavity of the 

 globular organ, which shortly after separates 

 from the mother plant. In this instance, as 

 in those above described, conjugation is pre- 

 ceded by the conversion of the green gra- 

 nular contents of the conjugating cells into 

 oil globules. The germ thus produced re- 

 tains its power of development for several 

 months, and gives rise to a new plant re- 

 sembling its parent in structure.* 



20. In Saprolegnia, which is morphologically 

 so closely related to Vaucheria, and like it, 

 in its earliest state of existence, produces 

 zoospores, we obtain, by the germination of 

 these zoospores, plants which produce repro- 

 ductive organs of an entirely different cha- 

 racter. These, when completely formed, 

 consist of spheroidal cells, each supported on 

 a cylindrical pedicle. Each contains in its 

 interior a number of round spores (from five 



* Karsten, Die Fortpflanzung der Conferva fon- 

 tinalis. (Bot. Zeit. 1852, 6 Stuck.) 



The process of which the details have been so 

 well described in the above memoir, was known to 

 Vaucher, and is mentioned by him in his " Histoire 

 des Conferves d'Eau douce" (p. 17.). See also Naegeli 

 (Vergl. Algensyst. p. 175.) ; Hassall (British Fresh- 

 water Algae, vol. i. p. 175.) ; and Thuret (Annales 

 des Sc. Nat. 2nd Ser. 1843), who gives a figure illus- 



are very much more numerous than the curved 

 tubular organs, a fact for the explanation of which 

 observations are as yet wanting. 



