The Various Substances Made by Plants iii 



other plants are of different aspect, as our plate so clearly illus- 

 trates (figure 35) ; but each kind exhibits characteristics peculiar 

 to itself, and in general it is true that no two species of plants 

 have grains exactly alike, while each 

 species has a kind distinctive of itself. 

 This fact has a practical value, because 

 experts with the microscope can thus 

 learn to recognize the starches of dif- 

 ferent plants at sight, and by this means 

 can detect adulterations in starchy foods 

 or drugs. Biologically, also, this indi- 

 viduality of the starches is of very great 

 interest, for it gives us a clear case in fig. 34.— a cell, highly magni- 

 which a well-developed specific character f^"' ^Z^i^^^ii;^, 

 exists without any regard to utility; for search grains embedded in 



living protoplasm. 



even the most radical adaptationist would 



hardly consider the forms of the deeply-buried and invisible 

 starch grains as useful in adapting the species to its environment. 

 And if an internal specific character can be useless, what need to 

 try to explain every external specific character as necessarily 

 useful? I am very well aware that this little digression will seem 

 without point to most of my readers, but I pray them to have 

 patience a little, for I have a good object. I am calling their 

 attention when I can to certain data which will later be useful 

 when we come to consider the subject of evolution. 



Cellulose. This substance is vastly abundant and prominent 

 in plants, for it is the material out of which they construct the 

 walls of their cells and therefore their entire firm skeletons. 

 The reader can obtain a good idea of pure cellulose by recalling 

 the fibers of cotton, the pith of woody stems, or some of the pur- 

 est unstarched paper, such as the filter-paper of the laboratories, — 

 all of which exhibit the distinctive cellulosian qualities of tough- 

 ness, elasticity and transparency. In some plants also, it is 

 stored up as a reserve food in the seed, when it appears as an im- 



