2IO NATURAL SCIENCE. March, 



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 Centnilblatt, Professor Mobius 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. Epilobium 

 angustifoliuin 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 



