17. SEXUAL REPRODUCTIVE ORGANS. 59 



process which is here termed fertilisation, the male gamete being 

 considered to fertilise the female ; product, an oospore. 



The gametes of isogamous plants, in those cases in which they 

 are set free from the gametangium and are free-swimming, are well 

 defined, ciliated, somewhat pear-shaped masses of protoplasm 

 destitute of a cell-wall (e.g. Botrydium, Ulothrix, Ectocarpus, etc.), 

 and are distinguished as planogametes. When, however, they are 

 not free-swimming (as in the Conjugate Algae) they have no defined 

 form nor are they ciliated. 



The gametes of heterogamoiis plants. The male gamete, when 

 the conditions are such that it must of necessity be free-swimming, 

 is generally a well-defined ciliated mass of protoplasm, termed a 

 spermatozoid. Spermatozoids occur in the heterogamous Green 

 and Brown Algse (e.g. Vaucheria. Volvox. Sphseroplea, (Edogonium, 

 Chara, Fucus), in the Bryophyta, in the Pteridophyta, and in a few 

 Gymnosperms. In the lower forms the spermatozoid is more or less 

 rounded or pear-shaped, somewhat resembling a planogamete of the 

 isogamous forms : but in the higher it is club-shaped or fila- 

 mentous, thicker at the posterior end, pointed at the anterior end 

 where the two or more cilia are borne, and more or less spirally 

 coiled. It has no cell-wall. 



When, owing to the proximity of the male and female organs at 

 the time of fertilisation, the male gamete has no considerable dis- 

 tance to traverse (e.g. most Phanerogams), it is not differentiated as 

 a spermatozoid, but is simply an amorphous cell without a cell-wall. 



The female gamete, or oosphere, is not ciliated, nor is it, as a 

 rule, set free, but remains in the female organ until after fertilisa- 

 tion : but in some Algse (e.g. Fucus), the oosphere is extruded from 

 the female organ before fertilisation. It is, generally speaking, 

 spherical in form, as its name denotes. It has no cell-wall. 



The gametes are developed from one or more mother-cells in 

 the gametangium. In isogamous plants, as a rule, each mother-cell 

 gives rise to more than one gamete, and commonly to a considerable 

 number (e.g. Botrydium, Ulothrix) ; but in Ectocarpus and some 

 other Phseosporous Algse, each mother-cell produces but a single 

 gamete. Whilst in the higher heterogamous plants the male 

 gametes are each developed singly from a mother-cell, in the lower 

 it is the rule that the male gametes are produced several together 

 from one mother-cell. The female gametes are developed singly in 

 the mother-cell, except in the Saprolegniacese among Fungi, and in 

 some genera of Fucacese (Pelvetia, Ozothallia or Ascophyllurn, 



