210 NATURAL SCIENCE. Marcu, 
nature of the reproductive organs: we have non-sexual flagellate 
zoospores; we have a rudimentary sexual process in the conjugation 
of zoogametes, flagellated motile bodies which closely resemble 
zoospores, but which have no power of germination without first 
uniting in pairs; and, finally, we have an advanced mode of sexual 
reproduction in which the male and female elements differ almost 
as widely as they do in the highest flowerless plants, the former being 
a minute motile multiflagellate antherozoid, the latter a much larger 
perfectly passive oosphere. But what is most interesting is that 
we find in the brown sea-weeds all sorts of intermediate conditions 
between these, leading us to the irresistible conclusion that all the 
higher developments of sexuality have had their first origin in the 
union of two motile flagellate masses of protoplasm between which 
there is no apparent differentiation; but that very early, as we ascend 
in the evolutionary scale, a differentiation sprang up between these 
two masses of protoplasm, which became gradually more and more 
marked as they developed into what we call male and female cells. 
The latter soon lost their flagellate character, became quiescent, and 
increased in size; the former gradually passed from a biflagellate 
zoogamete indistinguishable from a zoospore, to the multiflagellate 
antherozoid of the higher Cryptogams. The passage from these to 
the germinal vesicles in the ovule and the pollen-tube of flowering 
plants is somewhat more difficult to follow. 
The production or non-production of sexual organs in plants is 
very much a question of external conditions. In a recent paper 
in the Biologisches Centvalblatt, Professor Mébius has published a very 
interesting summary of our knowledge on this subject. An abun- 
dant supply of nutriment promotes the production of vegetative rather 
than of reproductive organs; hence the value of root-pruning in 
increasing the fertility of fruit trees. The conditions which, on the 
whole, favour the formation and fertility of the sexual organs are—a 
high temperature, with not too much moisture and not too much 
supply of nutriment, abundance of light, and, in the case of the 
fruit trees of temperate climates, a period of complete rest in the winter. 
Many of the fruit-trees of our climate will not flower in the tropics. 
The potato, which blossoms only sparsely with us, produces 
flowers every year in its drier native country of Chili. £pzlobiwm 
angustifolium will flower with us only in sunny situations, and the 
brighter the light the deeper the colour of the flowers. The produc- 
tion of flowers and fruit has an exhausting effect on the plant. A 
well-known illustration of this is the ‘‘century plant,” Agave ameri- 
cana, which flowers only once, when from 30 to 100 years old, and then 
dies. A very abundant fruit year is commonly followed by one of * 
comparative scarcity. 
A similar law prevails also in the lower forms of vegetable life; 
but with this difference, that we find there the much greater plasticity 
which is the great feature of a low type of life. Many of our 
