96 BOTANY. [chap. hi. 



it is true that in some species the taste is very pungent, 

 but this does not avail them against the nibblings of cer- 

 tain molluscs. In other species specialized strands of 

 cellulose are present for the purpose of strengthening the 

 cell-walls and to neutralize the strain exerted on the 

 tissues by the movement of the water ; others, again, 

 have special contrivances in the way of hoUowed-out 

 portions of the thallus or vegetative portion of the 

 plant that contain air for the purpose of enabling the 

 plant to float on the surface of the water, and thus ex- 

 pose itself to light in connection with the function of 

 assimilation. 



Notwithstanding the primitive nature of the Algae as 

 a group, we find the leading ideas of .plant life clearly 

 indicated within the group. In the simplest unicellular 

 or one-celled microscopic forms, the single cell is often 

 spherical, and even when we come to multicellula/r- spe- 

 cies consisting of numerous cells, the same idea of 

 solidity is present; but in the higher groups we find 

 this weak point rectified, and the substance of the vege- 

 tative portion flattened out into comparatively thin sheets 

 — leaves in fact in function — for the purpose of exposing 

 the greatest amount of area possible from a given quan- 

 tity of material, thus enabling the organism to assimilate 

 a greater amount of food, an indispensable necessity for 

 bhe well-being of the individual. In connection with 

 reproduction, or the continuation of the species in time, 

 there is a sequence from the primitive asexual or vegeta- 

 tive mode to highly differentiated sexual methods, which 

 are so arranged as to render possible the invigorating 

 influence of cross fertilization; and from the algss up- 

 wards, the great variety of forms occurring in plant life 



