SYSTEMATIC BOTANY 81 



a Pine tree, and a Rose as belonging to different grades ; 

 and that, for instance, a Toadstool, a Liverwort, a Harts- 

 tongue fern, a Yew tree, and a Lily form a similar series. 



These series of plants each represent the five principal 

 groups into which systematists have divided the families. 

 The scientific names of these groups are the Thallo- 

 phyta, the Bryophyta, the Pteridophyta, the Gymno- 

 sperms and the Angiosperms. In addition to these, 

 there are one or two important kinds of plants which 

 existed in past time, but which have since become ex- 

 tinct. Of these the Pteridospermae, mentioned already 

 in the chapter on Palaeontology, lie between the pterido- 

 phytes and the gymnosperms. 



Each of the five main groups are divided into a number 

 of divisions, sometimes called phyla, each of which is 

 composed of several families. 



The Thallophyta have the largest number of species 

 after the Angiosperms, and number about eighty 

 thousand species all told. They are all comparatively 

 simple in structure and have no differentiation into 

 true leaves, stems, and roots, and have no woody or 

 true vascular tissue. They have only spores and no 

 seeds, but some of them have an alternation of genera- 

 tions. In this case, in one generation reproduction is 

 by simple spores, and in the second it is by means of 

 a spore resulting from the fusion of two sexual cells. 

 This is not at all regular, however, and in many cases 

 it depends on the nutrition and other conditions, which 

 method of reproduction results. A large number of 

 the Thallophyta never produce other than the simplest 

 spores. A great proportion of these forms are very 

 small and simple and live in the protecting medium of 

 water. Such are all the small green algae- of the ponds 

 and streams, all the seaweeds, red, green, and brown, 



F 



