1326 
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 
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 do 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 
PHYSIOLOGY 
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 the young stem. 
1788 root-hairs, and also 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, 
