22 



KNOWLEDGE 



[Jancaby 1, 1808. 



direct evidence of its geological age. These questions are 

 outside our present object, but indicate how far the green 

 weed from the garden path might lead ug. 



The special feature we want to observe is the mode of 

 reproduction of the plant, and it will be found that it 

 propagates itself by two distinct methods.* 



In the first case there is an aggregation of the 

 protoplasm at the ends of certain threads, and in time this 

 specialized portion makes its way through the terminal 

 wall and swims about by means of vibrating cilia, 

 which occur in pairs all over its surface. In time this 

 liberated mass of protoplasm loses its cilia, settles down, 

 develops a cellulose wall, and passes into a resting stage. 

 Later on, it germinates and grows directly into a new 



the main axis. Their contents are, however, cut off from 

 the latter by a transverse wall or septum. The larger 

 inflated bodies contain each a rounded protoplasm mass 

 which is the oosphere or egg-cell. The narrower tubular 

 structures are the antheridia, and at the right stage will be 

 found full of minute antherozoids formed by repeated sub- 

 division of the protoplasm and nuclei. These anthero- 

 zoids or spermatozoids are minute oval bodies each with 

 a pair of cilia, by means of which they move. 



They escape from an aperture at the apex of the 

 antheridium, which in most species curves round so as 

 to approach the top of the oogonium.* The wall of the 

 latter becomes gelatinous at this point, and the antherozoids 

 pass through and effect the fertilization of the oosphere. 



A. — Vaucheria arersa. — Tlie lilameDt in the centre slio«s two Oogonia and two Antheridia. The Antlieridium on the 

 left is empty, and the fertilized Oosphere in the corresponding Oogonium has developed a thick wall. In the upper filament 

 the protoplasm is aggregated at the apex, and shut off by a septum prior to the formation of a Zoogonidium. B. — The 

 Coenoeytie Zoogonidium of Vnncheria passing out from the apex of a filament, c. — The Caenocytie Zoogonidium of Vaucheria, 

 showing numerous peripheral nuclei, with pairs of Cilia opposite each. D. — An Oogonium, with Antherozoids passing through 

 the mucilaginous apical area. E. — Antherozoids (or Sperinatuzoids). F. — Germination of an Oospore or Oosperm. 



Vaucheria plant. This process of renovation of physio- 

 logical energy in a special part of the protoplasm is termed 

 " rejuvenescence." 



For the other and more important method of repro- 

 duction, one must examine the small protuberances which 

 occur here and there on the sides of the threads. These 

 will be found to be tubular or oval outgrowths from the 

 filament enclosed by a cell wall continuous with that of 



* It should be noted the type of oogamoas reproduction liere 

 described occurs in Vaucheria only. In the other genera the process 

 of reproduction is in some cases still unobserved ; in others it takes 

 place by conjugation of similar, or slightly dissimilar, free swimming 

 " gametes." 



Subsequently the oosphere surrounds itself with a thick 

 protective wall, passes through a period of quiescence, and 

 in time germinates, growing at once into a new plant. 



Such is a brief summary of the life history of this 

 common but no less interesting plant. 



The type has been selected as affording a simple 

 example of oogamous reproduction ; and the important 



* The number and distribution of the oogonia and the form of 

 the antheridia differ in the various species. The one chosen for the 

 illustration is a fresh-water species, and was collected in a pond near 

 Croydon. The commoner V. sessilis. on damp earth, has the curved 

 antheridium ; as also V. hamata, V. racemosa, and others. The type 

 here shown is the simplest of all, and has not been figured in the 

 usual text-books. 



