1324 



PHYSIOLOGY 



PHYSIOLOGY 



green wood, and often as much as 95 per cent in the 

 pumpkin. 



The Food Supply of Salts from the Soil. Besides se- 

 curing from the soil its water supply, the plant must 

 secure in the same way all of its ash constituents, and 

 usually all of its nitrogen, as soluble salts. The salts 

 furnishing food are such well-known food ingredients, 

 or constituents of fertilizers, as potash compounds, 



1781. Sweet pea grown in 

 soil containing the tuber- 

 cle-forming organism. 



1782. A similar plant in a 

 soil freed from the tu- 

 bercle bacteria. 



phosphates, nitrates, etc. The various mineral elements 

 generally necessary for the plant are potassium, phos- 

 phorus, sulfur, calcium, magnesium, and iron in small 

 quantities. These, as well as other inessential elements, 

 are the constituents that remain in the form of ash when 

 the plant is burned in air. That each one of these ele- 

 ments, as well as nitrogen, is necessary for the full 

 development of the higher plant has been repeatedly 

 demonstrated. For this demonstration seedlings are 

 supported and grown in jars containing culture solu- 

 tions. One or more of the above elements may be left 

 out in certain cases to be compared with one in which 

 all are present, and it will then be found that growth 

 and development will soon be arrested where even one 

 necessary element is entirely absent. 



Nitrogen Sometimes Furnished by the Air. Plants 

 ordinarily get their nitrogen from the soil as nitrate of 

 soda, saltpeter, or other soluble salts. This is the rule, 

 and although the air contains about 75 per cent by weight 

 of the free gas nitrogen, it is in this form entirely inert 

 to most plants. Leguminous plants (Leguminosae) form 

 a great exception to this rule. On the roots of such 

 plants are found swellings or tubercles, as in Fig. 1781, 

 caused by the growth of parasitic bacteria. By the aid 

 of these bacteria, the plant is able to appropriate the 

 free nitrogen of the atmosphere and to thrive in a soil 

 almost free of nitrates. This has been demonstrated 

 repeatedly with plants in sand or water cultures. Utiliz- 

 ing the free nitrogen of the air, leguminous crops, 

 whether used as green manure or not, restore the land 

 by returning to it more nitrogen than is taken away. 

 They necessarily enter into any thorough system of 

 rotation upon weak lands, and represent an important 

 economic factor in horticulture, particularly in orchard 

 culture. 



The Association of Roofs with Fungi. Other plants 

 are also aided by an association of the root with certain 

 filamentous fungi. In connection with the roots of the 

 oak, beech and other woody as well as herbaceous 

 plants, this association of root and fungus (termed 

 mycorhiza) is of much aid in the absorption of solu- 

 tions from the soil. In such cases it is thought that the 



fullest development of the plant is dependent upon the 

 presence of the fungus. 



The Formation of Organic Stibstance.Next to the 

 elements of water (hydrogen and oxygen) carbon forms 

 the bulk of the plant substance. It is a constituent of 

 all starchy, sugary or nitrogenous (organic) products, 

 and of all woody fiber. It is, in fact, the element whose 

 presence indicates organic substances. When plants 

 are burned in an atmosphere more or less devoid of 

 oxygen, the bulk of the remaining charcoal is pure 

 carbon, for the ash present would be so small in volume 

 that it might be neglected. The ordinary green plant 

 obtains this carbon only from the carbon' dioxid of the 

 atmosphere. Carbon dioxid forms only .04 to .05 per cent 

 by weight of the atmosphere, and in order to enter into 

 the composition of the plant, it must first diffuse itself 

 into the tissues of the leaves and other green parts. The 

 leaf is so constructed that the diffusion of carbon dioxid 

 readily takes place. Numerous stomata lead from the 

 epidermal surfaces into the air spaces, which penetrate 

 all parts of the leaf, as in Fig. 1783. By means of this 

 system of communication, diffusion results, and the 

 carbon dioxid is brought into contact witn a large ab- 

 sorbing surface of living cells. Together with favor- 

 able growth conditions, however, the utilization of 

 carbon dioxid depends upon the leaf -green (chlorophyll) 

 and sunlight. The green chlorophyll absorbs a con- 

 siderable part of the rays of sunlight, and by means 

 of the energy thus provided carbon dioxid (CO 2 ) is 

 forced to unite with a portion of the absorbed water 

 (H 2 O). The union of these substances is accomplished 

 by an obscure process, but in such a way that eventually 

 carbon and the elements of water ( hydrogen and 

 oxygen ) are united in the form of a carbohydrate. 

 This substance is first readily demonstrable as starch, 

 of the composition C6Hi O 5 . In this process of 

 union, or synthesis, there is an excess of oxygen fur- 

 nished, and it is this oxygen which is thrown off by 

 the leaves. This process of uniting carbon dioxid and 

 water under the influence of chlorophyll and sunlight 

 in the living tissues is known as photosynthesis a 

 building up of higher substances by sunlight. The pro- 

 cess is also called carbon dioxid assimilation. Pho- 

 tosynthesis and respiration are, in a way, reverse 

 processes ; in the former carbon dioxid is absorbed in 

 sunlight and oxygen given off; in the latter, as pre- 

 viously noted, oxygen is constantly absorbed and carbon 

 dioxid given off. However, the ordinary plant, as a 

 whole, respires but feebly, while it assimilates carbon 

 very rapidly in sunlight. The result is that by night 

 a small amount of carbon dioxid is given off and by 

 day a large amount of oxygen. See Photosynthesis. 



The starch made in sunlight and stored in the cells of 

 the leaves is in the form of insoluble granules, like 

 commercial starch. In order to be utilized by the pro- 

 toplasm in growth, or in forming further plant sub- 

 stance, it must first be transformed, or digested. This 

 is constantly taking place in the leaves by means of an 

 enzyme called diastase. The diastase converts the starch 

 to a soluble substance, a form of sugar, and this sugar 

 may then be used immediately in building up more 

 complex organic compounds used by the protoplasm, or 

 the sugar may be transported to some storage organ 



1783. 



A cross-section showing the cell structure of a 

 typical leaf. 



of the plant, such as root (dahlia), stem (potato), or 

 thickened leaf, and there be reconverted to starch or 

 changed to some insoluble or more complex product. 

 This translocation of the starch formed in the leaves 

 by day may be so thoroughly effected during a single 

 night that none will remain as starch by the next day. 



