For these reasons the first and most imperfect of the great provinces or branches of the vegetable kingdom, the division of the Algæ, or Tangles, must be of special interest to us. But, in addition, there is the interest which this group offers when viewed by itself. In spite of the exceedingly simple composition of their constituent cells, which are but little differentiated, the Algæ show an extraordinary variety of different forms. To them belong the simplest and most imperfect of all forms, as well as very highly developed and peculiar forms. The different groups of Algæ are distinguished as much by size of body as by the perfection and variety of their outer form. At the lowest stage we find such species as the minute Protococcus, several hundred thousands of which occupy a space no larger than a pin’s head. At the highest stage we marvel at the gigantic Macrocysts, which attain a length of from 300 to 400 feet, the longest of all forms in the vegetable kingdom. It is possible that a large portion of the coal has been formed out of Algæ. If not for these reasons, yet the Algæ must excite our special attention from the fact that they form the beginning of vegetable life, and contain the original forms of all other groups of plants, supposing that our monophyletic hypothesis of a common origin for all groups of plants is correct. (Compare p. 83.) SYSTEMATIC VIEW _Of the Six Branches and Eighteen Classes of the Vegetable Kingdom._ ========================================================================================== _Primary Groups_ | | | _or Sub-Kingdoms_ | _Branches or Clades_ | _Classes_ | _Systematic Name_ _of the_ | _of the_ | _of the_ | _of the_ _Vegetable Kingdom._ | _Vegetable Kingdom._ | _Vegetable Kingdom._ | _Classes._ ------------------------+-----------------------+----------------------+------------------ { { 1. Primæval algæ 1. _Archephyceæ_ { { (Protophyta) { { { I. { 2. Green algæ 2. _Chlorophyceæ_ A. { Tangles { (Chloroalgæ) =Thallus Plants= { _Algæ_ { Thallophyta { { 3. Brown algæ 3. _Phæophyceæ_ { { (Fucoideæ) { { { { 4. Red algæ 4. _Rhodophyceæ_ { { (Florideæ)