THALLOPHYTES. 



I. SEXUAL REPRODUCTION". In all cases that have been fully examined 

 we find sexual reproduction initiated by a process of fertilisation. This consists in 

 the coalescence of two cells, masses of protoplasm, a male and a female, which 

 may be of different form and size ; in the simplest cases they are not distinguishable 

 in outward appearance from one another; the two extremes are however con- 

 nected by intermediate forms. The two masses of protoplasm which thus coalesce 

 are called gametes, and may either possess the power of independent motion 

 ( ( planogamefes ' or zoogametes\ or be destitute of it. The chief forms of sexual 

 reproduction are the following * : 



1. Conjugation and Formation of Zygospores. Two cells of similar if not 

 always the same constitution coalesce and produce a reproductive cell, termed 

 a zygospore, which lies dormant for a considerable time and then germinates, and 

 either produces directly a plant of the same kind as that on which the conju- 

 gation took place, or gives rise first of all to a number of brood-cells. 



The process in the formation of zygospores wears a different aspect according 

 to the nature of the conjugating cells. The simplest case is that of the conjugation 

 of swarm-cells (planogametes) dis- 

 covered by Pringsheim (Fig. i A); 

 pairs of these bodies as they swim 

 about come in contact with one 

 another at their hyaline anterior 

 ends, and gradually coalesce into 

 a spherical primordial cell, which 

 becomes invested with a cell-wall 

 and increases in size ; after a time 

 it breaks up into motile cells, from 

 which proceed plants of the original 

 kind. The process of conjugation 

 in Spirogyra is more complicated ; it 

 will be found represented in Fig. 25. 

 There the conjugating cells have 

 firm cell-walls, and put out processes 

 towards one another; these unite and form a canal through which the living 

 contents of the one cell pass over into the other and there coalesce with its con- 

 tents; the protoplasmic body formed by this coalescence becomes invested with 

 a cell-wall and is a zygospore, which by direct germination produces a spirogyra- 

 filament. The formation of the zygospore of a Zygomycete is explained by 

 Fig. i -, in this case two cells, entirely alike and motionless, coalesce by a 

 normal process of growth, and a portion only of the united contents, cut off by 

 transverse walls, produces the thick-coated zygospore, which germinates after a 

 lengthened period of repose. 



FIG. i. Different modes of conjugation and formation ot zygospores. 

 A pairing of the swarm-cells of Pandorina. B formation of zygospores 

 of Piptocfphalis. The successive stages in the development are shown in 

 A by Arabic, in B by Latin numerals. After Pringsheim and Brefeld. 



1 More detailed information with regard to the facts here stated will be found further on 

 in the special description of the Algae and Fungi. For the knowledge of the facts here adduced 

 we are indebted to the labours of De Bary, Pringsheim, Thuret and Bornet, Nageli, Brefeld, 

 and others. 



