PLANT PHYSIOLOGY 247 



plants, following the photosynthesis of carbohydrates, have 

 been mainly katabolic in their final results. We shall now 

 consider those metabolic changes in carbohydrate material 

 that are anabolic. 



Cell Wall. — We have spoken of the fact that the plant cell 

 contains all three of the essential organic compounds, carbo- 

 hydrates, fats and proteins, as constituent parts of the cell 

 protoplasm. In addition to the cell protoplasmic contents we 

 have the cell wall, and this consists largely, if not wholly, of 

 carbohydrates in the form of cellulose. In all cases it is pro- 

 duced by the conversion of the translocated sugars into cellulose, 

 by reactions undoubtedly similar to those by which starch is 

 formed in leaves and seeds. 



Cellulose. — In the young living cells of plants the cell wall 

 is probably pure cellulose and it is somewhat different in char- 

 acter from the cellulose of mature and old cells found in the 

 stems, branches and trunks of the plant. Oftentimes the 

 cellular structure takes on peculiar forms and produces the 

 fiber which is contained in the boll of the cotton plant and in 

 the stems of grasses, flax, hemp, etc. As the plant grows, 

 especially in the case of large trees, the pure cellulose of the 

 young cell walls becomes thicker and harder, due to the im- 

 pregnation with gums, resins, lignins, pectins, pentosans, etc. 

 Cell walls thus hardened give to the cellular structure the 

 character of wood, which is much tougher and stronger than 

 young cellular structures. In all these cases, whether in young 

 living cells or in old woody cells, the cellulose of the cell wall is 

 truly the building material of the plant and is not reserve food 

 material. It is into such building material that a great part of 

 the photosynthesized carbohydrates is eventually metabolized. 

 When we consider the immense amount of cellulose thus formed 

 in the case of large trees, and realize that while the plant is 

 forming all of this body substance, it is, at the same time, using 

 photosynthesized carbohydrates as direct energy food for the 

 cells and also forming large amounts of reserve starch food, 

 we get some idea of the magnitude of the process of photo- 



