278 PLANT PHYSIOLOGY 



While the ordinary stem is not especially adapted for food 

 storage, certain regions, notably the rays, cortex, and pith, may 

 accumulate large amounts. The rays connect the cortex and the 

 pith so that movement from one region to the other is facilitated. 

 In addition, the wood parenchyma cells scattered through the 

 xylem are commonly found gorged with food. 



Of the specialized underground stems, tubers and rhizomes are 

 of exceptional importance. The former, found on the ends of 

 underground stems as in the potato, store very large quantities 

 of food thus enabling them to serve as organs of vegetative repro- 

 duction. The rhizome is a thick, fleshy, underground stem, of 

 which examples are to be found in the iris, fern, sedges, etc. Al- 

 though the bulb as seen in the onion, hyacinth, tulip, etc., is mostly 

 composed of modified leaves which act as storage organs, it does 

 contain a small stem and is generally classified as such. The corm 

 differs from a bulb in being a solid stem without the leaves and is 

 found in the dasheen (Colocasia), Indian turnip (Arissema), Crocus, 

 and Gladiolus. 



Fleshy storage roots are very common. Among them may be 

 mentioned the beet, carrot, parsnip, radish, sweet potato, dahlia, 

 etc. Seeds are also rich in foods and are among the most important 

 of the storage organs. 



In the cases above mentioned the food is stored in the organ 

 for the direct benefit of the plant which produced it. The food is 

 stored either for the benefit of the plant itself at the beginning of 

 the next growing season, as is generally the case of the food stored 

 in stems and roots, or for the benefit of the young embryo of the 

 next generation, as in the case of seeds. Man, however, as well as 

 other herbivorous animals, interferes and frequently robs the plant 

 for his own uses, as witness the bulk of our food, which comes 

 from roots, stems, and seeds. 



Fruits, on the other hand, contain stored food which is only of 

 indirect value to the plant which produced it. The survival value 

 of fruits has been the attraction they offer to animals, which thus 

 aid in the scattering of the seeds. Hence fruits, from the point of 

 view of the economy of the plant, belong in an entirely different 

 category from the stems, seeds, and roots. 



Relation of Plant Duration to the Storage Organ. — There is a 

 distinct relation between the nature of the plant and the kind of 

 organ in which the food is stored. Annuals, which last only one 



