THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



19 



soluble form. This is accomplished by chemical 

 substances called ferments, the presence of small 

 quantities of which makes possible a large amount 

 of chemical action. They are of the greatest im- 

 portance in both constructive and destructive 

 processes. 



Plants without chlorophyll (saprophytes, living 

 in decaying matter, and parasites, deriving nour- 

 ishment from living organisms) are unable to make 

 elaborated food. Some parasites that have chlo- 

 rophyll, as mistletoe, have this power. Insectivo- 

 rous plants secure an extra supply of nitrogenous 

 food from the capture of insects. Plants of the pea 

 family secure nitrogen from the air by means of 

 bacteria living in their roots; this relation between 

 two plants is known as sjrmbiosis. 



Growth. — Growth may be best illustrated by con- 

 sidering the growing tip of a stem. Here we may 

 distinguish three stages : 



(1) The formative region, where cells are con- 

 stantly dividing and new organs are being formed. 



(2) The elongating region, where the cells 

 expand by absorbing large quantities of water. 

 This comes next to the formative region. 



(3) The maturing region, where the cells no 

 longer expand but assume their characteristic form 

 and markings. 



In the first of these stages the cell is filled with 

 protoplasm. As the absorption of water continues, 

 drops are formed in the protoplasm ; these coalesce 

 to form a single large drop (vacuole) that occupies 

 almost the entire cell. This condition persists in 

 the later stages. 



Growth depends very much on temperature, 

 increasing rapidly up to about 30° C; above this 

 it diminishes. It depends, also, on an adequate 

 supply of water, food and air. Light, especially 

 the blue rays, generally checks the growth. 



Movement. — Movement in plants may be pro- 

 duced by the contraction, or other movement, of 

 the protoplasm, as in animals. It is usually due, 

 however, to unequal growth of opposite sides of an 

 organ (e. g., the opening and closing of flowers). 

 The movements of the leaves of the sensitive plant 

 and of clover are due to changes in the turgidity 

 of cells. 



Irritability. — Irritability is the power of re- 

 sponding to stimuli. When a leaf folds up at a 

 touch, we say that the touch acts as a stimulus. 

 The amount of energy needed to execute the move- 

 ment is much greater than was imparted to the 

 leaf by the act of touching it. The stimulus sets 

 free stored energy, just as a touch on an electric 

 button may explode a powder magazine. Among the 

 stimuli to which plants respond may be mentioned 

 light, gravity, heat, chemical substances, electric- 

 ity, strains and contact. In general, the plant 

 responds by bending toward or away from the 

 source of stimulus or by changing the rate of 

 growth. 



Reproduction. — The process of fertilization in 

 higher plants has already been described. This is 

 called sexual reproduction, because it results from 

 the union of two nuclei, a male and a female. 



In addition, we find asexual reproduction, in 



which no such fusion takes place. The propaga- 

 tion of plants by cuttings, leaves, tubers, roots 

 and bulbs furnishes familiar illustrations of this. 

 Simple division, as in bacteria, or budding, as in 

 yeast, are also methods of asexual reproduction. 

 Asexual reproduction by means of specially modi- 

 fied single cells, called spores, is found in ferns, 

 mosses, molds, bacteria and other plants. 



In all plants, down to and including the mosses 

 and liverworts, there is a regular alternation of 

 sexual and asexual generations. A sexual genera- 

 tion (prothallium) arises from the asexual spore 

 (e. g., of a fern) and bears sexual organs. After 

 fertilization, the egg produces a plant that is 

 called the asexual generation, because it produces 

 no sexual organs, but only asexual spores, which, 

 in turn, give rise to the sexual generation. In the 

 fern, the sexual generation is of microscopic size, 

 while the asexual (spore-bearing) generation is the 

 familiar fern plant. 



The environment of the plant. 



The needs of the plant are like those of the ani- 

 mal, namely, water, food, light, air and warmth. 

 The plant resorts to endless contrivances to secure 

 a sufficiency of these, as well as to protect itself 

 against an excess of any of them. It constantly 

 adjusts itself to external conditions in order to 

 make the most of its circumstances. Were it not 

 able to do so it would soon perish. We may briefly 

 consider the chief factors of the environment. 



Water. — Nothing affects the plant more than the 

 water-supply. The effect of dry conditions is best 

 seen on desert plants, which show the following 

 modifications : (1) Reduced surface secured by 

 partial or total suppression of leaves, as in cacti. 

 The discarding of leaves in winter is an adapta- 

 tion to the dry conditions then obtaining. Even 

 when there is water in the ground the roots can- 

 not absorb it, because of the low temperature ; (2) 

 reduced evaporation secured by thicker epidermis, 

 coverings of wax and of varnish ; (3) reduced 

 evaporation secured by rolling the leaf, as in 

 grasses, or placing it in a permanently vertical 

 position, as in iris ; (4) storage of water in the 

 thickened stem or leaf ; (5) reduction in the num- 

 ber of stomata and sinking them in depressions ; 

 (6) hairy coverings of the leaves ; (7) increased 

 woody fiber; (8) smaller air-spaces; (9) longer 

 palisade cells of the leaf. 



Aquatic plants show the opposite characteristics, 

 having large surfaces, thin epidermis, no waxes, 

 resins or hairs, very little woody fiber, very large 

 air-spaces, and poorly developed palisade cells in the 

 leaf. Stomata occur only on the surfaces exposed 

 to air, but are there numerous. 



The size of every part of the plant is increased 

 by an abundance of water. The large-celled spring 

 wood is an illustration of this. The small-celled fall 

 wood is formed under much drier Conditions. 



Growing plants in a saturated atmosphere pro- 

 duce, curious modifications ; a cactus may thus be 

 made to produce leaf-like organs, and gorse pro- 

 duces leaves instead of thorns. On the other hand, 

 a potato grown with a minimum amount of water, 



