IIow the Plant Grows. 5 



similar to starch and sugar in general composition and originates 

 from them. In the dense wood of trees the cell walls are thick, in 

 some cases nearly filling the entire cell. In the more tender twigs 

 and leaves they are less dense, while in the still softer portions, 

 such as fruits and seeds, they are thin and delicate. More or less 

 mineral matter or ash is built into the cell walls of plants, being 

 especially abundant in the bark of trees, as is shown when such 

 material is burned. 



The pentoses and pentosans are carbohydrates with 5 atoms of 

 carbon in the molecule, in place of 6 as in the sugars and starches. 

 The pentoses correspond to sugars, and the pentosans to starches 

 and cellulose. The pentosans are largely associated with cellulose 

 in the more woody portions of the plant, being abundant, for ex- 

 ample, in wheat bran and corn cobs. 



4. Vegetable fats and oils. — In some eases the plant stores car- 

 bon in the form of fat, which is solid at ordinary temperatures, or 

 of oil, which is liquid. Such storage is entirely possible since fats 

 and oils are formed from the same elements that exist in the carbo- 

 hydrates. In vegetable fats and oils the molecules are composed 

 of a larger number of atoms than are those of the sugars, and the 

 proportion of carbon is greater, as the following formulae of three 

 common vegetable oils or fats show : 



Stearin ^^.i^^iio^a 

 Palmitin QJIasOe 

 Olein Cs^Hio^Oe 



Vegetable oils and fats give off more heat on burning than do the 

 carbohydrates, because they contain relatively more carbon. Oils 

 and fats most abound in the seeds of plants and represent carbon 

 energy stored in condensed form. When seeds containing oil, as 

 the flax seed for example, begin to grow, the oil is changed over 

 into products which nourish the growing plantlet the same as do 

 the products of starch in ordinary seeds. 



5. Nitrogenous compounds. — We have learned how in the life- 

 holding protoplasmic masses of the green parts of plants, especially 

 their leaves, the carbohydrates and fats are formed from the ele- 

 ments of carbon dioxid and water by the energy of the sun. To 

 these life centers of protoplasm, with their green coloring matter, 

 holding sugar and starch, the sap brings nitrates and other mineral 

 salts gathered by the roots from the soil. Thru the union of the 

 elements of the nitrates and other salts with those of the starches 



