INTRODUCTION 



of the lower plants it is the only form of reproduction known. A 

 number of non-sexual types of reproduction are known in plants, 

 the two principal being either by spores, usually single cells, 

 which become detached and grow into new individuals ; or by bud- 

 ding, or the formation of branches, which, on becoming detached, 

 already show the principal organs complete ; indeed, it is not easy 

 to say what limits the individual in plants, as there is such a repeti- 

 tion of parts. A tree, for example, may be compared to a stock 

 of Coral, with its multitude of similar individuals, rather than 

 to a highly organized individual like an Insect or Vertebrate. If a 

 branch is severed from the tree, it may under proper conditions 

 develop roots, and establish itself as a new stock. This never occurs 

 among the higher animals, where the power to restore lost parts is 

 exceedingly limited, and new individuals must always be produced 

 from special sexual reproductive cells. 



Sexual Reproduction. Sexual reproduction consists in the produc- 

 tion of a new individual by the fusion of two cells, generally the 

 product of different individuals. There is extraordinary similarity 

 in the character of the sexual cells of plants and animals, as well as 

 in the phenomena connected with their development and union. 

 This is the more striking because it is clear that sexuality has devel- 

 oped quite independently in widely separate groups, and there are 

 still existing a number of classes of plants which show all stages of 

 the process. In the simplest form of sexual reproduction the cells 

 are quite similar, but there is usually a well-marked separation 

 into male and female cells, distinguished by differences hi size, 

 and in many instances by the motility of the male element 

 (sperm), which is a free-swimming, ciliated body, while the much 

 larger female cell egg-cell or ovum is usually passive. The 

 sperm-cell penetrates the egg, and its nuclear substance mingles with 

 that of the egg, which is thus stimulated into further growth, and 

 produces, directly or indirectly, the new generation. 



Biology 



Animals and plants agree so closely in their cell-structure and the 

 essential life-functions nutrition, respiration, and reproduction 

 that these points may be made the subject of biological study irre- 

 spective of whether the organisms concerned are plant or animal, 

 all coming equally within the domain of Biology. However, since 

 the peculiar animal or vegetable characters become manifest very low 

 down in the scale of development, it is possible to relegate most 

 organisms to either the animal or vegetable kingdoms, and we there- 

 fore recognize two coordinate branches of Biology, Zoology and 

 Botany. 



