Conjugation occurs in algae and fungi.  A typical case is that of Spirogyra.  This is an alga with its cells in long filaments.  Two contiguous cells of two parallel filaments push each a little projection from its cell-wall toward the other.  When these meet, the protoplasm of each of the two cells contracts, and assumes an elliptical form—­it undergoes rejuvenescence.  Next an opening forms where the two cells are in contact, and the contents of one cell pass over into the other, where the two protoplasmic bodies coalesce, contract, and develop a cell-wall.  The zygospore thus formed germinates after a long period and forms a new filament of cells.

Another example of conjugation is that of Pandorina, an alga allied to the well-known volvox.  Here the conjugating cells swim free in water; they have no cell-wall, and move actively by cilia.  Two out of a number approach, coalesce, contract, and secrete a cell-wall.  After a long period of rest, this zygospore allows the whole of its contents to escape as a swarm-spore, which after a time secretes a gelatinous wall, and by division reproduces the sixteen-celled family.

We now come to fertilization, where the uniting cells are of two kinds.

The simplest case is that of Vaucheria, an alga.  Here the vegetative filament puts out two protuberances, which become shut off from the body of the filament by partitions.  The protoplasm in one of these protuberances arranges itself into a round mass—­the oosphere or female cell.  The protoplasm of the other protuberance divides into many small masses, furnished with cilia, the spermatozoids or male cells.  Each protuberance bursts, and some of the spermatozoids come in contact with and are absorbed by the oosphere, which then secretes a cell-wall, and after a time germinates.

The most advanced type of fertilization is that of angiosperms.

In them there are these differences from the above process:  the contents of the male cell, represented by the pollen, are not differentiated into spermatozoids, and there is no actual contact between the contents of the pollen tube and the germinal vesicle, but according to Strashurger, there is a transference of the substance of the nucleus of the pollen cell to that of the germinal vesicle by osmose.  The coalescence of the two nuclei within the substance of the germinal vesicle causes the latter to secrete a wall, and to form a new plant by division, being nourished the while by the mother plant, from whose tissues the young embryo plant contained in the seed only becomes free when it is in an advanced stage of differentiation.

Perhaps the most remarkable cases of fertilization occur in the Florideae or red seaweeds, to which class the well-known Irish moss belongs.

Here, instead of the cell which is fertilized by the rounded spermatozoid producing a new plant through the medium of spores, some other cell which is quite distinct from the primarily fertilized cell carries on the reproductive process.