108 ON THE PLANT-CELL. 



The conditions mentioned are of the highest importance as regards 

 the life of the entire plant. Carbonic acid, ammonia, and water, con- 

 stitute the chief nutritive matter of the cell, which, however, takes it up 

 in various ways. Those cells which are in contact with fluid receive all 

 the three substances at once. In this, consequently, must the most active 

 assimilating process take place. Those cells which are partially in con- 

 tact with the air obtain, it is true, on the one side, all necessary elements 

 dissolved in water, but they can also receive, on the other side, carbonic 

 acid and ammonia from the atmosphere. At the same time they give 

 out into the atmosphere a larger or less quantity of water, by the loss of 

 which their juices are concentrated, and this concentration again main- 

 tains the endosmosis. This enables us to explain how it is that after the 

 bursting forth of their leaves plants no longer abound with such very 

 watery sap, and yet continue the process of assimilation with greater 

 energy. The perfect solution is conveyed farther by the endosmosis. 

 The salts held in solution by the water and the inorganic constituents in 

 general, upon which the chemical forces of the cell have little influence 

 or none at all, are carried with the water unchanged through all the 

 cells until they reach the surface of the cells at which evaporation takes 

 place. At this point, they of necessity gradually accumulate in larger 

 quantity, which accounts for the greater residue after incineration left 

 by the leaves, green bark, &c. The water evaporated from the cell, 

 like all water when evaporating, carries away with it a small quantity 

 of non-volatile substances, on which account the water perspired by 

 plants is never quite pure *, but is impregnated more with organic than 

 with inorganic (less volatile) matter. 



53. By the association of many cells, and the reciprocal in- 

 fluence thence set up, certain modifications are produced in the 

 life of the individual cells which have been in part previously 

 considered. To these modifications is probably to be referred, 

 in part, the formation of new distinct layers, and the spiral arrange- 

 ment of the material constituting these layers connected therewith. 

 This part of the subject also embraces the peculiar construction of 

 air-vesicles between two contiguous cells, upon which the forma- 

 tion of the pores appears to depend. 



What refers to this subject has been already discussed ( 17.). In no 

 isolated cell, in no cell previous to its association with others into a 

 tissue, do we find spiral deposit-layers ; nor, moreover, do we observe in 

 any such the air-vesicles on the outer wall to which the canals of the 

 pores correspond internally. It appears that the canals of the pores of 

 two contiguous cells always correspond in sucli a way that they com- 

 mence from an air-vesicle of this kind, or from a space in the common 

 wall corresponding to it. We are acquainted with but few exceptions 

 to this, which, however, demand further investigation. In Juniperus 

 Sabina there occur in the bark thick-walled, four-sided, prismatic cells, 

 the pore-canals of which regularly run only towards the four inter- 

 cellular passages, which in this instance, and in a tissue which elsewhere 

 presents no intercellular passages, appear to represent the air-vesicles 

 above described. The same thing is seen in the parenchyma of the 

 petiole in Cycas (vide p. 45.). In the epidermis-cells of several plants, 



* Sennebier, Phys. veg. t. i. p. 79., and many others. 



