1913-1914.] IQ9 



certain Algae, Mosses, Ferns, and Flowering Plants, it at once 

 becomes evident that in the complete life-cycle of any of these 

 there appear two individuals. One of these individuals is char- 

 acterised by possessing sexual organs, and may therefore be called 

 the " sexual plant ; " the other reproduces by bodies known as 

 spores, and hence is called the " spore plant." Sexual and spore 

 plants bring each other into the world alternately. This altem 

 ation in the appearance of these two unlike individuals is known 

 as the alternation of generations. In an Alga such as Coleochaete, 

 the sexual plant consists of a green radiating filamentous structure, 

 which lives in fresh water. The spore plant, which arises from 

 the fertile egg of the sexual plant, consists of eight cells only, each 

 of which produces the motile spore. The spore plant is also 

 aquatic. In the Moss (e.g., Polytrichum commune) the sexual 

 plant consists mainly of a leafy stem. At the apices of the leafy 

 stems the sexual organs appear. The spore plant is nothing but 

 a long stalk, bearing an apical sporecase. Whereas Coleochaete 

 was wholly aquatic the Moss is terrestrial, but depends upon 

 external fluid water for the fertilisation of its eggs. The Fern 

 spore plant is familiar to all. It has a stalk bearing compound 

 leaves and a root system. The sexual plants are small, flat, heart- 

 shaped plants, which produce their sexual organs upon their under 

 surfaces. As in the Moss, fertilisation of the eggs of the Fern is 

 dependent upon external fluid water. The Pine-tree illustrates 

 the situation in flowering plants. The tree itself is the spore- 

 bearing plant. The spores are of two kinds, male and female. 

 The male spores are the pollen grains, and the female spores are 

 to be found one in each young seed. Within the pollen grain is a 

 minute little male plant consisting of two cells, which produces 

 one male organ. Within the female spore is a mass of tissue in 

 which female organs are embedded. The vast majority of Algae 

 are aquatic. The vast majority of Flowering Plants are terrestrial. 

 Thus, plants in the course of evolution have changed from aquatic 

 to terrestrial habits, and as plants changed from aquatic to terres- 



