iuhkf oirr/jNi'] of vk(;i:tai!Lk piiyhiolooy 231 



off at the ends, and its length be exactly measured. Let the stringy 

 external sheath tlien he strijiped off, and at onee let lioth the central 

 cellular column and one or two of tlio external strips be measured. It 

 will be found tliat tlie pith has considerably lengthened, while the 

 librous strips are somewhat shorter than the piece of leaf stalk origi- 

 nally measured. Before separation, tlion, the pith must have been 

 compressed, the external tissues stretched. Tissue tensions add rigid- 

 ity to stems, petioles, etc. Variations in tissue tensions give rise to 

 curvatures of organs, such as the bending of the stem toward the 

 light. 



537. Solid components of the plant body. — By solid components 

 is meant here all the matter left when water has been entirely driven 

 off by heat at somewhat above the boiling temperature of vi'ater. 

 This includes cell walls, dried living substance (protoplasm), starch, 

 sugar, and other formed matters in the cells, and small amounts of 

 mineral matters ordinarily held in solution in the juices of the plant 

 or deposited in the tissues in crystalline form. 



538. Amongst these, the organic constituents are composed almost 

 solel)' of the four chemical elements — carbon, hydrogen, oxygen, and 

 nitrogen. Organic matters belonging to the class carbohydrates — as 

 sugar, starch, cellulose — and fats, include in their composition only 

 the first three of these elements; they lack nitrogen. Nitrogenous 

 organic compounds — as those that make up protoplasm — contain all 

 the four elements named, and in addition, usually a small amount of 

 sulphur and phosphorus. 



539. The nature of the mineral matters held in the plant is found 

 when the dried plant has Vieen Imrned and the ash has been chemically 

 analyzed. In burning, carbon and hydrogen are united with oxygen 

 from the atmosphere and pass away in a gaseous form. Organic com- 

 ponents of the plant body are therefore broken up. The ash that is left 

 is entirely inorganic. In such ash, from various plants, has been 

 found a large part of all the known chemical elements, including even 

 the rarer metals. Most of these elements occur accidentally, being 

 absorbed with soil water. But certain of the chemical elements are 

 absolutely necessary to the healthy growth of every green plant. These 

 are six in number; viz., sulphur, phosphorus, potassium, calcium, 

 magnesium, iron. 



540. Source of the elements. — Thus there are, including the 

 four elements before named as chiefly making up organic substance, 

 in all ten elements which must be furnished the growing plant. Each 

 of these is received by the plant in a combined form. Carbon comes 

 from the atmosphere, combined wdth oxygen, as carbonic acid gas. 

 All the other needful substances come from the soil. Hydrogen and 

 oxvgen come together, as water. Nitrogen is brought in under the 

 form of a soluble nitrate, or one of the ammonia salts, in the absorbed 



