MICROSCOPIC FOKMS OF VEGETABLE LIFE. 271 



connect this group with the preceding in linear series, but in the form 

 of discoidal fronds. In this tribe we meet with a form of multiplication 

 by zoospores aggregated into macro-gonidia, 1 which reminds us of the 

 formation of the motile spheres of Volvox ( 239), and which takes place 

 in such a manner that the resultant product may vary greatly in number 

 of its cells, and consequently both in size and in form. Thus in, Pedi- 

 astrum granulatum (Fig. 161), the zoospores formed by the subdivision 

 of the endochrome of one cell into gonidia, which may be 4, 8, 16, 32, or 

 64 in number, escape from the parent frond still inclosed in the inner 

 tunic of the cell; and it is within this that they develop themselves into 

 a cluster resembling that in which they originated, so that whilst the 

 frond normally consists of sixteen cells, it may be composed of either of 

 the just-mentioned multiples or sub-multiples of that number. At A, is 

 seen an old disk, of irregular shape, nearly emptied by the emission of its 

 macro-gonidia, which had been seen to take-place within a few hours pre- 

 viously from the cells a, b, c ? d, e ; most of the empty cells exhibit the 

 cross slit through which their contents had been discharged; and where 

 this does not present itself on the side next the observer, it is found on 

 the other. Three of the cells still possess their colored contents, but in 

 different conditions. One of them exhibits an early stage of the subdivi- 

 sion of the endochrome, namely, into two halves, one which already ap- 

 pears halved again. Two others are filled by sixteen very closely-crowded 

 gonidia, only half of which are visible, as they form a double layer. 

 Besides these, one cell is in the very act of discharging its gonidia; nine 

 of which have passed forth from its cavity, though still enveloped in a 

 vesicle formed by the extension of its innermost membrane; whilst 

 seven yet remain in its interior. The new-born family, as it appears im- 

 mediately on its complete emersion, is shown at B; the gonidia are actively 

 moving within the vesicle; and they do not as yet show any indication 

 either of symmetrical arrangement, or of the peculiar form which they 

 are subsequently to assume. Within a quarter of an hour, however, the 

 gonidia are observed to settle-down into one plane, and to assume some 

 kind of regular arrangement, most commonly that seen at c, in which 

 there is a single central body surrounded by a circle of five, and this again 

 by a circle of ten; they do not, however, as yet adhere firmly together. 

 The gonidia now begin to develop themselves into new cells, increase in 

 size, and come into closer approximation (D); and the edge of each, espe- 

 cially in the marginal row, presents a notch, which foreshadows the pro- 

 duction of its characteristic * horns. ' Within about four or five hours after 

 the escape of the gonidia, the cluster has come to assume much more of the 

 distinctive aspect of the species, the marginal cells having grown-out into 

 horns (E) ; still, however, they are not very closely connected with each 

 other; and between the cells of the inner row considerable spaces yet 

 intervene. It is in the course of the second day that the cells become 

 closely applied to each other, and that^he growth of the horns is com- 

 pleted, so as to constitute a perfect disk like that seen at F, in which, 

 however, the arrangement of the interior cells does not follow the typical 

 plan. 2 



271. The varieties which present themselves, indeed, both as to the 



1 Solitary zoospores or micro-gonidia have been observed by Braun to make 

 their way out and swim away; but their subsequent history is unknown. 



2 See Prof. Braun on "The Phenomenon of Rejuvenescence in Nature," pub- 

 lished by the Ray Society in 1853; and his subsequent Memoir, " Algarum Unicel- 

 lularum Genera nova aut minus cognita," 1855. 



