HELIOTROPISM. 677 



This is evidently — in the plant-cell just as in the engine — because 

 things are in the condition to react to slight stimulus. Thus, 

 when a ray of light falls upon a motile swarm-spore, and it swims 

 toward the illuminated side of the drop of water in which it is 

 confined, the undulations of the ether may he held to have caused 

 a more or less continuous change in the molecular structure of 

 the protoplasm ; energy is liberated, and ciliary motion in a cer- 

 tain direction is the final resultant. In the same way, when the 

 sun rises in the morning, rays fall upon the stems of the sunflow- 

 ers, intimate structural changes take place in the cell-protoplasm, 

 and, through mechanical contrivances which will be mentioned 

 later, a slow curving toward the light is effected. 



This remarkable instability of protoplasm — and the writer 

 craves the privilege of considering it only as a chemical com- 

 pound of astonishing complexity — is of deep interest when con- 

 sidered in its relation to groivth. Upon this something must be 

 said. The growing part of a plant is, as we know, only the living 

 part. Heart-wood is always dead wood, and is incapable of react- 

 ing to the external world except as unorganized matter might. 

 Furthermore — and this need scarcely be mentioned, since the rudi- 

 ments of botanical knowledge have become so wide-spread — the 

 whole mass of living, growing tissue is made up of cells more or 

 less crowded together, more or less individual in their forms and 

 functions, but all of the same general plan of structure. If one 

 could imagine the Capitol-dome at Washington completely filled 

 with a densely crowded mass of toy balloons, each balloon dis- 

 tended with water, and containing within the water, usually sur- 

 rounding most of it, a sac-like piece of sponge, it will be a fair idea 

 of what a growing-point would be like if seen upon a sufficiently 

 large scale. The phrase " growing-point " will be understood to 

 have the technical significance, meaning the extreme tip or apical 

 area of a bud or shoot. Each cell-wall contains its cell-sap, or 

 cell-fluid, and its cell-protoplasm, which was compared to the 

 sponges. The protoplasm is, of course, the only essential living 

 part, and the others are but elaborations and mechanisms by 

 which the complicated cell-life, as part of an organic whole, is 

 possible. Or the appearance of a growing-point might be com- 

 pared to the mound of small bubbles which may be blown in a 

 bottle half filled with suds. Hundreds of bubbles, each full to 

 bursting of air, press each other in every direction, and constitute 

 a more or less conical and coherent mass of bubbles. It is here 

 that the important point is to be noticed — 



Groiuih is possible only when the cells are in a state of tension. 



As one may easily discover, a flabby leaf will not increase in 

 size, and a limp and flaccid stem is equally incapable of growth. 

 In other words, growth of a plant-cell is like the growth of a bal- 



