20 



THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



exposed to light, assumes a cactus-like habit, with 

 no leaves and with very short internodes and thick- 

 ened stems. 



The water-supply directly influences the produc- 

 tion of flowers and fruit. Aquatic plants cannot as 

 a rule produce flowers under water. Land plants 

 with abundant water-supply run to stem and leaf, 

 and produce little fruit. Cutting oif the water sup- 

 ply at the proper time greatly increases the pro- 

 duction of fruit, and also makes it sweeter and of 

 higher flavor. By irrigating properly, we may con- 



The reach for light of a tres on the edge of a wood. 



trol both the quantity and quality of the crop. An 

 excess of water soon kills the plant by suffocating 

 the roots. 



Light. — The effect of light on the plant is very 

 similar to that of dryness, and in the case of desert 

 plants the strong light increases the effects due to 

 lack of water. Plants that prefer the sun are 

 known as sun-plants (grasses), while those that 

 can grow only in shade are known as shade-plants 

 (ferns). The latter have longer, thinner leaves, 

 usually of paler color. A similar difference may 

 often be observed between exposed and shaded 

 leaves on the same individual plant. The exposed 

 leaves have thicker epidermis, longer palisade cells, 

 smaller air-spaces and fewer stomata. 



Both leaves and branches arrange themselves 

 with reference to the direction of the light, and 

 the same is true to a large extent of flowers. This 

 is well illustrated by plants that grow near houses 

 so that they are shaded on one side. A further 

 illustration is the different arrangement of leaves 

 on upright and on horizontal branches of the same 

 plant. Excessive light produces "sunscald" and 

 other bad effects. Some leaves avoid this danger 

 by assuming a vertical position. On the other 

 hand, absence of light produces marked effects. 

 Chief of these is the pale color (etiolation) which 

 is so noticeable in celery that has been blanched 

 by being covered from the light, or in potatoes 



that have sprouted in darkness. The stem is 

 usually weak and spindling, while the leaves, in 

 dicotyledons at least, remain small ; hairs and even 

 prickles tend to disappear in darkness. With weak 

 light the colors of flowers are much less brilliant, 

 and the production of both flowers and fruit is 

 seriously checked, even when there is sufficient 

 food present for their formation. 



The reach for light is well marked wherever 

 plants are crowded. About the edge of a forest, 

 the trees branch on the outward side (Fig. 43). In 

 the midst of the forest they shoot straight up. In 

 the open field they branch on all sides and remain 

 low. When two or three trees grow close together, 

 they branch mostly in opposite directions These 

 adaptations are equally marked in bushes and herbs. 

 Food. — The food requirements of plants are very 

 different ; some grow best on poor soils, others on 

 rich soils. In general, starving a plant causes it to 

 flower and fruit more quickly 

 but to produce a less abundant 

 crop. Over-feeding creates a 

 tendency to produce stem and 

 leaf at the expense of fruit. 

 It also greatly increases the tendency to 

 produce monstrosities. Both these effects 

 are especially produced by an over-supply 

 of nitrogen. Abundance of water acts in 

 much the same way as abundance of food. 

 Over-supply of nitrogen may be corrected^ 

 to a certain extent, by the application of 

 potassium, which tends to check the over- 

 production of vegetative parts < and bring 

 about the development of fruit. Some ex- 

 periments seem to indicate that phosphorus 

 also directly favors fruit formation. 

 Lime is valuable not only as a food, but it helps 

 to make other mineral food available. It hastens" 

 the decomposition of humus, sweetens sour soil and 

 improves the texture of clay soils by its floccu- 

 lating action. It also acts as an antidote to the 

 poisonous action of magnesium, when the latter is 

 present in large quantities.. Some plants are found 

 only where lime abounds, while others cannot toler- 

 ate it except in small amounts. 



If any nutritive substance in the soil be reduced 

 to a minimum, the effect on the plant is much the 

 same as if all the nutritive substances were like- 

 wise reduced ; this is known as the " law of the 

 minimum." Consequently, the application of fer- 

 tilizer containing an element deficient in the soil, 

 may give results out of all proportion to its cost. 



It is possible in water cultures to determine 

 very closely the effect of excluding various neces- 

 sary elements. For example, it is thus found that 

 when iron is lacking, practically no chlorophyll is 

 formed. The facts so gained have not as yet been 

 applied to the soil to any great extent. 



The root has a "selective action" in that it 

 takes up from the soil certain elements, to the 

 partial or total exclusion of others. Thus, from a 

 solution of sodium nitrate, it takes nitric acid, 

 leaving the sodium. A cereal crop takes from the 

 soil only one-fourth as much potash and only half 

 as much nitrogen as root crops. This is one reason 



