96 ORGANIC STRUCTURE. 



Seed-bearing- stems are always exhibited in the air, 

 but the lower parts of them are, in many instances, 

 prolonged downward in the soil, as Dducus carrbta, 

 and Cbchlearia armor acea*, or extended horizontally 

 therein, as Agropyrus repens\> and as are those of 

 aquatic plants in the mud, as Nelumbium specibsum\. 

 Fig. 29. 



An aquatic plant extending its sterns in the mud. 



The bulbous stem has been already described. 

 Tuberous stems, as the turnip, are casual enlargements 

 of the pith, covered with the proper integument of 

 the stem, but also greatly thickened and pulpy. 

 This, as well as the incrassated stems of Dducus, 

 Pastinaca, Beta, and others, are, for the most part, 

 only the effects of cultivation. The tubers of Heli- 

 dnthus, and Solanum tuberbsum , are the abbreviated 

 and engrossed points of subterranean stems of the 

 plants. Of such subterranean stems there are modi- 

 fications, and they may readily be distinguished from 

 what are properly called tuberous roots ; the former 



* Horse radish. ( Couch-grass. 



| Water-lily. Jerusalem artichoke and potato. 



