OF VEGETABLE TISSUES. 203 



the matters composing the blood ; we have to suppose a force unlike gravity and affinity, as it 

 exists in inorganic matter. The development of organic bodies is not a precipitation j Mul- 

 der was right in this assertion : but he resolves the organic formations into peculiar activities, 

 which belong to oxygen, hydrogen, nitrogen and carbon ; their combinations are of a me- 

 chanical class. So, too, we may say, the peculiar heat, termed animal heat, and generated by 

 chemical combinations in the body, is of a class not identical with the species, culinary or 

 solar heat. But it is extremely difficult to discern the difference between animal and culinary 

 or solar heat. There is a class of forces resembling vital forces, not so unlike them, at least 

 according to some philosophers, as the chemical : the electro-magnetic attractions, for example. 

 The poles of a battery attract acids and alkalies to them ; the acids go to the positive pole, 

 while the negative receives the alkalies. For one, however, I never have been able to re- 

 cognize in the phenomena of life electro-magnectic attractions, as forces to which formative 

 functions are due. Looking at the elementary organ, the cell, we see that it must be instru- 

 mental and active, in separating from the blood the secretions of the body, bile, urea, gastric 

 fluids, etc. The cell membrane is the instrument, but whether it is an electrical force residing 

 in the membrane is an unsettled question : that it is electrical is not proved. But we must be 

 careful and not overlook the properties of the fluids acted upon, as I have elsewhere stated. 

 Bile exists in the blood, and there must exist in bile a property, sui generis, by which it is • 

 possible for separation to be effected by the liver ; or, the organ and the matter separated must 

 stand in certain relations to each other. The liver can not separate urea, because urea has not 

 the same relation to the bilary force that bile has ; that is, the result is not due entirely to the 

 organ. We still regard it as the instrument, an instrument adapted to the properties of the 

 bile, otherwise it would be no instrument at all. Spongy platinum acts in a particular manner 

 upon hydrogen ; it condenses it, till finally it burns ; nitrogen thrown upon spongy platinum 

 exhibits no such phenomena, for there is absent a mutual adaptation by which condensation 

 and combustion can ensue. 



I have, thus far, been speaking of matters pertaining to development of cells. A question, 

 in this connection, comes up directly, how organized beings grow ; or what is growth 1 It is 

 evidently an increase in the number of cells, and their enlargement. The pervading fluid of 

 the cells contains matter for new ones, and these continue to be produced, till the number re- 

 quired by the organ to which they belong is completed. The cells, from that moment, cease 

 to be produced, and growth is effected by their enlargement. There is an elongation of the 

 axis upward and downward, the elongation taking place mainly at the extremities of branches. 

 The roots extend, or grow, entirely by the production of cells at the apex, or end of the root, 

 which terminates in a fringed organ, called a spungiole ; the extremity is thereby pushed for- 

 ward by the newly developed cells. This elongation of the root continues during the life of 

 the plant. At first the growth is downwards, and in many trees, as the pine, large tap roots 

 are formed ; but the progress in this direction is limited, and soon the growth is horizontal. 



But trees increase in diameter. When in the spring, in temperate climates, the individual 

 tree is permeated "by fluids, an organizable fluid is poured out from the entire surface of the 



