218 



REPRODUCTION, VEGETABLE (VEGETABLE OVUM). 



12. In the process of the formation of zo- 

 ospores in Saprolegnia, we have an intermediate 

 step between that of the zoosporous Algae on 

 the one hand, and that of a class of plants which 

 is usually placed among the Fungi on the other. 

 I allude to the Fungi included in the claps 

 Cystosporeae of Leveille ; on the intimate 

 structure of this, as well as of many other 

 allied groups, there are as yet but few re- 

 searches. We have, however, enough in the 

 beautiful monograph of Cohn, on Pilobolus, 

 to enable us to discover that it is structurally 

 more closely allied to the Alga? than to the 

 Fungi. We shall take Pilobolus as an illus- 

 trative example. 



13. Pilobolus has an ephemeral existence. 

 The spore germinates about mid-day; the plant 

 grows till evening, ripens during the night. 

 In the morning the spore-case bursts, and the 

 whole disappears, leaving scarcely a trace of 

 its former existence. 



In correspondence with the future mode of 

 life of the plant, the spore-cell displays in its 

 germination, a tendency to development in two 

 opposite directions, by the formation of two 

 sacculations, the first, cylindrical the root; 

 the second, ellipsoidal the stem. Shortly 

 afterwards the young plant is seen to consist 

 of two cells, of which the inferior is elongated 

 and branched at its lower extremity root-cell; 

 while the superior is ellipsoid, and acuminated 

 above. The former contains a quantity of pro- 

 toplasma, which lines, as a distinct layer, the 

 internal surface of its wall. The first change 

 which is observed consists in the accumulation 

 of this protoplasma towards the apex of the 

 cell, at which point the membrane buds out, so 

 as to form a bead-like head. Within the cavity 

 of this organ the future spore-case, further 

 accumulation takes place, until it is entirely 

 filled with a coloured granular material; while 

 the rest of the cell, from which it is as yet undi- 

 vided, contains only a clear fluid. The pro- 

 cess is completed by the formation of a septum 

 just as in Vaucheria, which takes place early 

 in the morning. This is immediately followed 

 by the "cleaving" of the protoplasma, and its 

 division into numerous small cells, which are 

 the future spores. As the plant reaches the 

 termination of its existence, the cell on which 

 the spore-case is supported, enlarges at its 

 upper part from the increase of its fluid con- 

 tents ; the septum is pushed upwards, and 

 presses on the contents of the spore-case. At 

 last in the course of the forenoon, the tension 

 of the wall of the spore-case becomes so great 

 that it gives way at its junction with the sup- 

 are pear-shaped, and possessed of cilia. These ob- 

 servations I have been altogether unable to confirm, 

 and am inclined to believe that the escape of the 

 zoospores in the spheroidal form is to be attribute's 

 to an arrest of development, as in all cases which I 

 have observed, the total disappearance of the spores 

 has supervened shortly afterwards. Anton de 

 Bary, Beit. z. Kentnisse der Achlya prolifera. Bot. 

 Zeit. 28 St. 1852. For further information on 

 Achlya, see Unger, Linnasa, 1843, p. 129. ; Nageli 

 Zeit. f. wiss. Bot. B. i. H. 1, 2. Pringsheim, Nova 

 Acta Ac. L. C. 1851. 



porting cell with such force, that it is thrown 

 like a miniature bomb for several inches.* 



14*. The Fungi which agree in their develop- 

 ment with the species above described, are 

 limited in number, and belong for the most 

 part to the genera Pilobolus and Ascophora 

 (Mucor). The formation of the spore differs 

 entirely from the process of strangulation, 

 which Schleiden considers as characteristic of 

 the Fungi. On the other hand, the analogies 

 between Pilobolus and Vaucheria are of the 

 closest kind ; even the ephemeral periods 

 observed in the development and ripening of 

 the reproductive apparatus, being the same. 

 The root-cell of Pilobolus the inferior of the 

 three of which the whole plant is composed, 

 is as permanent as the tubular frond of a 

 Conferva. From it emanate tubular, unjointed 

 root-like processes, from the upper surface of 

 which spring out at intervals young spore- 

 cases, in every respect similar to the first- 

 formed plant. These creeping rootlets con- 

 stitute the vegetative system of the plant, 

 which, like that of the Fungi, is perennial. 



15. Reproduction by conjugation. From the 

 number of the observations which, during the 

 last few years, have been made on the sub- 

 ject of the phenomena of conjugation, no less 

 than from the variety of the conditions 

 under which they have presented themselves. 

 we are bound to assign them an important 

 place in a systematic description of the repro- 

 ductive process. Decaisne included in his 

 group Synsporea? all the Alga? in which the 

 phenomena in question were then known 

 to present themselves namely, the genus 

 Zygnema and its allies, along with Closterium, 

 which last, for the same reason, he separated 

 from the Desmideae. The beautiful researches 

 of Mr. Ralfs have taught us that all the 

 genera of the Desmideae conjugate in the same 

 manner as Closterium. More recently analo- 

 gous phenomena have been observed in the 

 Vaucheriacea3, and in that remarkable plant 

 Saprolegnia ferox, which so closely resem- 

 bles Vaucheria in every respect, except its 

 green colour. We shall describe in succes- 

 sion each of the examples which have been 

 mentioned. 



16. Among the Desmideae, conjugation has 

 been more frequently observed and described, 

 and was known to take place at an earlier 

 period in Closterium, than in any other genus. 

 The earliest description is that of Morren, which 

 is to be found in the Bulletins of the Aca- 

 demy of Brussels, for 1836, and is among the 

 most accurate that we possess. The crescent- 

 shaped cell forming the frond of Closterium 

 is, as in the Desmideae, composed of two 

 similar halves, to the plane of junction of 

 winch its long axis is perpendicular. It 

 differs from other genera in the absence of a 

 median constriction, the junction being only- 

 indicated by a faint line in the external mem- 



* Cohn, Die Entwickelungsgeschichte des Pi/<>- 

 bolus crystaHiiws. Nova Acta Ac. L. C. p. 4%. 

 1851. 



