COMPARISON OF THE GROWTH OF CRYSTALS, PLANTS, AND ANIMALS. 413 



between organs and crystals, in that the enlargement of a crystal takes place by the 

 deposition of new substance at its surfaces ; whereas the plastic substance serving for 

 the growth of the organ is present in its interior, and is conveyed to it internally from 

 other organs. Organs grow not by apposition, like crystals, but by intussusception : 

 their growth is an extension acting from within. 



Nevertheless we have cause to assume that, apart from all external matters, even 

 the growth of the organs of the plant — namely, their elementary structures, cell-walls, 

 protoplasm, and nucleus — depends fundamentally on forces which correspond essen- 

 tially with the forces of crystallisation ; only that here, since the supply of substance 

 capable of being organised takes place from within, complications which are not easily 

 intelligible present themselves. If the comparison here pointed out meets the case, it 

 must be mentioned that only the smallest particles of a cell-wall, protoplasm, or 

 nucleus (the MicellcE of Naegeli) grow after the manner of crystals ; and thus, not the 

 external form of an entire plant-organ, but the invisible structure of its elementary 

 constituents might be immediately referred to the forces of crystallisation. Naegeli 

 has undertaken, in a long series of important researches, to follow up and explain 

 further these (his own) ideas. His theory of growth by intussusception is concerned 

 in the first place with the question how the smallest invisible elements of the organised 

 parts of plants grow\ It is clear, however, that only one who is familiar with all the 

 details of the internal and external structure of vegetable organs can be in a position 

 to enter into the questions here raised. Moreover, the theory of intussusception has 

 by no means as yet attained to that degree of clearness which renders it possible 

 to explain in detail from it the growth of vegetable organs — leaves, roots, &c, I 

 renounce, therefore, in the interest of the reader, even an attempt in this direction, in 

 the conviction that the mere mention of this problem suffices to indicate the difficulty 

 of the task with which the theory of growth has to do : meanwhile we will keep 

 more to the surface of the matter, and attempt to make clear those processes of the 

 growth of the organs which are immedia'tely perceptible to the senses. 



The growth of the plant has been so far compared with that of the crystal. In 

 the essential points of comparison animals agree with plants. Comparisons between 

 the two kingdoms of organised beings, however, again yield some important differ- 

 ences. Apart from a few of the most simply organised forms, animals attain by means 

 of their grow'th a definitive condition, of such a kind that in a fully grown animal 

 all the organs are fully developed, each contributing by its physiological labour to the 

 maintenance of the entire body : the completely developed animal consists of com- 

 pletely developed organs, each performing its functions to the utmost. The case is 

 quite otherwise with plants. They are never completely developed. It is true we 

 find on every living plant completely developed parts, but besides these there are 

 always present in addition the rudiments or beginnings of new organs, capable of 

 further development. 



A plant without those growing-points which effect further development is no 

 longer a normal plant. Moreover, even the cut-off parts of living plants are very 

 often able to originate new growing-points of like kind. 



The continuity of animal life (apart from some plant-like lower forms) is 

 brought about by means of continually repeated reproduction, the single individuals 

 having normally only a limited and often very short period of life. Something 



