340 PHYSIOLOGY OF GROWTH AND CONFIGURATION 



through the stem to the tubers. The same author accomplished similar results with 

 rhizome-bearing forms, as well as with tuberiferous plants. If the organs in which 

 starch usually accumulates are removed, this substance may be made to accumulate in 

 organs not usually serving as places of accumulation. If it usually accumulates in a 

 subterranean tuber, it may be made to accumulate in aerially formed tubers, or in 

 roots, etc. 



2. Influence of Internal Conditions on Development. — Each part of the plant body 

 influences the development of other parts, and these internal influences (called correla- 

 tions) are in many cases so marked that they cannot be readily overcome, if that is 

 possible at all, by altering the external conditions. Correlations appear to be due to 

 small amounts of specific substances produced in the cells and influencing the growth 

 and development of other cells that are often situated at a great distance. Such 

 growth-controlling internal secretions are well known to occur in animals and they have 

 been called hormones, or chemical messengers. It seems probable that similar sub- 

 stances occur in plants. 



3. Reproduction. — Reproduction is a special kind of growth. In sexual reproduc- 

 tion, among the various organs appearing in a mature individual are the reproductive 

 organs, in which are produced the reproductive cells (eggs and sperms). The forma- 

 tion of these organs, like that of other organs, is controlled by internal and external 

 conditions acting together. Thus, Klebs was able to secure oogonia or antheridia, 

 or both, by proper treatment of the right form of Vaucheria. With the right treat- 

 ment, this alga could also be made to live indefinitely, reproducing by zoospores and 

 by branching, without the formation of sexual organs at all. Sperms of algae, mosses, 

 and ferns, are attracted to the archegonia or oogonia of the same or of a similar species 

 by certain substances that diffuse (dissolved in water) from these organs. The 

 sperms are said to be positively chemotropic toward these substances. Pollen tubes of 

 higher plants are similarly attracted by substances formed in the stigma, etc. ; in this 

 case the tubes bend because of chemotropism, this being a growth bending, due to un- 

 equal elongation on the opposite sides of the organ. 



In some cases egg cells develop into new individuals without fertilization, this 

 phenomenon being parthenogenesis. By proper treatment of the unfertilized eggs, 

 parthenogenesis may be artificially induced in some forms in which it does not usually 

 occur. 



It is common in plants for parts of the body to separate from the rest and develop 

 into new individuals, without any fusion of cells. This is asexual reproduction. In 

 Vaucheria, for a simple example, the terminal portion of the protoplasm of a filament 

 becomes a zoospore, which, after moving about for a time, simply grows into a new 

 plant. By proper treatment Klebs was able to produce zoospores in this alga at will, 

 or to grow the plant without their being formed. — Higher plants reproduce asexually 

 by means of outgrowths of various kinds, such as tubers, bulbs, etc. A branch, a 

 Teaf, or a piece of root, may thus act as a reproductive organ. Many cultivated plants 

 are propagated by cuttings; advantage is taken of asexual reproduction in order to 

 maintain valuable varieties, which would be lost in many cases if propagated by seeds. 



Related to propagation by cuttings is propagation by grafting or transplantation. 

 A bud or branch {scion) cut from one plant and inserted on another {stock) develops on 

 the new stem or root, retaining all the characters of the plant from which the bud or 

 branch was taken. Widely different parts of plants may be joined by transplantation. 

 In general, transplantation is possible only when closely similar species or forms are 

 involved. 



