INTRODUCTION 5 



other living things may be almost entirely differentiation. In 

 most cases, however, both occur together. 



A description of the growth of a seed plant will help to make 

 clear the way in which growth occurs, the way in which increase 

 in size and differentiation take place. 



A seed plant normally originates from a single cell in a part 

 of the pistil of the flower. This cell is the megagamete or egg. 

 It unites with a second, usually much smaller cell, the micro- 

 gamete or sperm cell, the process of uniting being fertilization. 

 The fused product of these two cells is the zygote or fertilized 

 egg* It is usually microscopic in size and structurally much 

 like an amoeba, though not motile. The zygote increases in size, 

 but not indefinitely. It soon divides into a chain of cells. One of 

 these, the proembryo cell, differs from the rest in the fact that 

 it continues its growth and forms the many-celled embryo plant. 



ply the direction of growth may be reversed time after time. Many roots shorten 

 or contract longitudinally and thus draw buds down into the soil. The small 

 tubers of Arum maculatum which are found at a depth of 2 cm. are subsequently 

 drawn into the soil to a depth of 10 cm. by a shortening of the root. Cells which 

 have differentiated may under certain conditions dedifferentiate, that is, become 

 embryonic again. It might be more accurate, therefore, to define growth as a 

 change in size or differentiation rather than as an increase in them and to look 

 upon it as a more or less reversible process. To be called growth the change should 

 be more or less permanent. The change in size which a strip of potato undergoes 

 when immersed in salt water or fresh water is not thought of as a growth change. 

 * Here at the very beginning of the growth of such a living thing we are 

 faced by a problem, and the same problem exists for other living things which 

 originate from a fertilized egg or zygote. As a general rule an unfertilized egg 

 will not grow. Not until after it has joined with the sperm cell does it begin 

 that ordered series of events which we call growth and which eventually result 

 in the mature plant or animal. Why is this? What sort of influence does the 

 microgamete exert upon the megagamete which starts it to growing? We cannot 

 discuss these questions adequately here. We can only say that the eggs of some 

 kinds of plants and some kinds of simple animals grow without being fertilized 

 (parthenogenetically) and that the eggs of some other plants and animals which 

 normally require fertilization have been stimulated by mechanical injury or by 

 treatment with acids or solutions of various salts to growth without uniting with 

 a sperm cell. Jacques Loeb succeeded by such artificial parthenogenesis in starting 

 unfertilized frogs' eggs to grow and in producing a fatherless frog. See Lillie, 

 Problems of Fertilization ; Pearl, Biology of Death. 



