16 BOTANY. [chap. i. 



the wide range under which protoplasm retains its indi- 

 viduality and performs its functions with regard to 

 temperature^ certain minute Algaa flourish in hot springs, 

 whereas other species find their home replete with every 

 comfort on the surface of snow or ice in high latitudes 

 or elevated regions, thus existing under a range of tem- 

 perature varying from freezing-point to within a few 

 degrees of the boiling-point of water. The above illus- 

 tration shows the possible range of protoplasm under 

 certain conditions, but the protoplasm of any given 

 species that has become diflferentiated and stereotyped 

 in one groove is always confined within much narrower 

 limits with regard to all surrounding forces, as tempera- 

 ture, pressure, amount of moisture, etc., and, as already 

 stated, the range becomes more restricted in proportion 

 to the amount of difierentiation and division of labour 

 presented by an individual; consequently, while many 

 of the lower forms of Algae are met with living under 

 very varying external conditions, the higher types of 

 plant life are divided into groups that are confined, more 

 especially by temperature and amount of moisture, to 

 certain portions of the earth's surface. 



The pioneers of plant life — Alg^ — were aquatic, and 

 the gradual extension of these forms to the dry land neces- 

 sitated an unforeseen amount of division of labour not 

 anticipated by the earliest members attempting this change 

 of habitat ; eventually, however, after many false starts, a 

 proper method was hit upon, that is, the living organism, 

 by a certain amount of modification of its already exist- 

 ing unit of structure, the cell, was enabled by degrees 

 to establish itself on dry land, and from these primitive 

 microscopic forms of algal life the whole of the Vegetable 



