1326 



PHYSIOLOGY 



PHYSIOLOGY 



concerned in the conduction of the digested or leaf- 

 formed foods to other parts. 



Seed Production. So far as we know, the ultimate 

 function of a plant in nature is to produce seeds or to 

 reproduce its kind. It matters not how far the horti- 

 culturist may have diverted this natural function in 

 particular instances, in general the sum of the physi- 

 ological activities is directed to seed-production. Much 

 energy is directed to the development of form and color 

 in the flower, also of fragrance and odor, and there are 

 deep-seated physiological processes connected with pol- 

 len and ovule production, with pollination, fertilization 

 (see p. 579), and the subsequent development of the 

 seed. 



Seeds are, as a rule, richer in nitrogenous matter 

 than other parts of the plant. Likewise, in phosphorus 

 and magnesium salts there is a marked increase in the 

 seed. ,Of these last-named substances, there is a migra- 

 tion, as it were, from the older parts to the region of 

 seed formation, and finally to the seed. On the other 

 hand, the salts of lime gradually increase in quantity in 

 the older tissues, particularly in the old assimilatory 

 tissues. 



The Living Protoplasm the Seat of Vital Action. 

 Physiological activities cannot be thoroughly studied by 

 the use of the plant as a whole or by the use of the 

 organs as particular parts of a complex whole. The 

 final seat of all the plant activities resides in the living 

 protoplasm of the cells composing the plant. Except as 

 serving purely mechanical purposes, the old heart wood 

 and bark of trees are inactive, and they contain no liv- 

 ing substance. They are made merely of the hardened 

 walls of cells which once constituted living parts. The 

 actual living parts, such as the leaves, buds, flowers, 

 fruits, and young wood, are composed of living cells. The 

 most essential part of a living cell is the protoplasm, a 

 semi-fluid, viscid substance which constitutes the living 

 material in all organisms. A definite layer of the pro- 

 toplasm surrounds the inner surface of the cell wall, 

 and protoplasmic strands radiate throughout the cell, 

 in which is also differentiated a denser and absolutely 

 essential part termed the nucleus. In addition the cell 

 contains an abundance of cell sap, or water, holding in 

 solution certain food substances. The cell wall is a 

 mechanical support, and as a physiological agent it is 

 quite dependent upon the protoplasm. In conjunction 

 with the wall layer of protoplasm, the cell sap absorbs 

 water osmotically from weaker solutions outside, and by 

 the same process solutions are passed from cell to cell 

 .and diffused throughout the growing parts. When trans- 

 piration is proceeding it is some of this water of the cell 

 sap which is given off through the leaves into the air. 

 As a result of this loss of water the protoplasm con- 

 tracts away from the cell wall and the rigidity (turgor) 

 of the cell is lost. Thus the cells and the tissues lose 

 strength, and the plant becomes flaccid and wilted. 



It is by means of the chlorophyll, but it is not the 

 chlorophyll alone which has to da with the formation of 

 starch from carbon dioxid. The chlorophyll is imbed- 

 ded in the living substance, forming definite chlorophyll 

 bodies; and it is only when associated with living matter 

 that it can perform its functions. 



The Plant is Affected by External Conditions : It is 

 Irritable. When a seed is put to germinate, the first 

 requisite is that it shall imbibe water and swell. Oxy- 

 gen is at hand, and if the necessary temperature pre- 

 vails the protoplasm is awakened to activity, and new 

 growth is incited. The protoplasm increases in bulk in 

 existing cells, and then cell division begins. At first 

 the embryo draws upon the seed for its food supply, 

 and is able to establish itself in the soil. A differentia- 

 tion into tissues and organs having different functions 

 has already occurred. Moreover, as soon as growth be- 

 gins, the influences of external agencies assert them- 

 selves. The first shoot does not wander about in the 

 soil, but, directly against the force of gravity (nega- 

 tively attracted), it directs itself upward. In an exactly 

 contrary manner, the first root attracted by the stimulus 

 of gravity (positively attracted) directs itself down- 

 ward. Only the overthrow or overbalancing of gravity 

 by some superior stimulus can prevent this reaction. 

 If a pot containing a seedling be placed upon its side, 

 the stem will actually curve when some growth has 



already occurred, bending itself directly upward, as 

 shown in Fig. 1787. The root will form a curve in its 

 growth, and again grow downward. The response of 

 growing organs to the stimulus of gravity is called geot- 

 ropism. Geotropism acts upon the active growing part 

 and by means of the living protoplasm. 



The relation of the plant to light, or the light stim- 

 ulus, is one of the most pronounced phenomena in 

 nature. In a dark chamber 

 young shoots will direct 

 themselves or grow directly 

 toward light admitted through 

 a small slit. Note how the 

 seedling bends toward the 

 light in Fig. 1788. If exposed, 

 the roots would direct them- 

 selves in a contrary manner. 

 Even the mature leaves of all 

 plants will turn or lean toward 

 the source of light. This may 



1787. 1788. Young seedling showing 



Negative geotropism of root-hairs, and also stem 



the young stem. bending towards the light. 



be well observed outside when the sun is low, and at 

 any time of day with a window garden. An interesting 

 case of the response to light is to be found in the wild 

 lettuce (Lactuca Scariola), which is known as a com- 

 pass plant. In sunlight this plant holds its leaves in 

 a vertical plane, one row of leaves pointing north and 

 the other south. This provision may be to avoid the full 

 rays of the midday sun, and yet to secure the best ad- 

 vantage of the less intense forenoon and afternoon sun- 

 shine. The response of plant organs to the stimulus of 

 light is known as heliotropism. 



In the same way plant organs will be stimulated to 

 grow towards or away from air (aerotropism), a certain 

 degree of moisture (hydrotropism), a definite tempera- 

 ture (thermotropism), nutrient substances or other 

 chemical agents (chemotropism) mechanical irritation 

 (thigmotropism) and other stimuli. In all of these ways 

 the plant is active and irritable. In all cases it is the 

 active protoplasm which* is concerned in determining 

 the nature of the response. 



Temperature has a marked effect upon all living pro- 

 cesses and it deserves particular mention. It may limit 

 either by too great heat or too intense cold each of the 

 particular vital activities. There are three critical tem- 

 peratures for growth, a maximum or higher tempera- 

 ture, a minimum or lower temperature beyond which on 

 either side no growth takes place, and the optimum, or 

 that intermediate grade which brings to the best devel- 

 opment all of the faculties of the plant. Sometimes the 

 optimum as reckoned by the amount of growth would 

 not correspond to the optimum for flower or seed pro- 

 duction, a fact well recognized in greenhouse culture. 

 The growth optimum may also be a temperature at 

 which the plant is more readily attacked by parasitic 

 diseases. Particular varieties or species vary greatly as 

 to their susceptibility to disease at different tempera- 

 tures. Often it is of more value to know the tempera- 

 ture at which the general sanitary conditions for a plant 

 are an optimum, rather than to know the optimum for 

 growth alone. The absorption of water by the root- 

 hairs, the manufacture of starch by the leaves, transpira- 

 tion, and other processes are to a large extent depen- 

 dent upon the temperature. Hot, dry winds of the sum- 

 mer-time often cause serious injury to trees, owing to 

 the rapid transpiration from the leaves. In dry seasons 

 this is very likely to occur with the Norway maple. 

 Fig. 1789 represents an injury of this kind. As a rule, 



