VIII.] MINUTE STRUCTURE. in 



Thunbergia. Examine also the pollen-masses of Orchi- 

 dacece and Asclepiade^e referred to in Part II. 



5. We have spoken of cells as containing fluid. So they 

 do, as long as they continue to take part in the work of the 

 plant. But in the trunks of trees the cells forming the 

 older wood sometimes become so very thick-walled that 

 they cease to do any work, and, indeed, may be said to 

 have no room left to do it in. 



6. Take some active, sufficiently transparent cells, such 

 as you find upon the margin of any young leaf bearing short 

 hairs upon its surface, and, removing a morsel of the leaf 

 without injuring the hairs upon it, place it in a drop of 

 water upon a glass slide under the microscope. You ob- 

 serve that each hair is simply a cell of the surface of the 

 leaf which has grown out into the air. Now, if you add 

 some fluid that will kill the cell, such as a drop of spirits of 

 wine, you will find, after allowing it time to act, that the 

 contents of the cell separate from the wall of the cell and 

 collapse, lying as a loose sac or irregular mass in the middle. 

 We may, therefore, distinguish cell-contents from cell-wall. 

 And the distinction is an important one, since all the real 

 work of the plant is done by the cell-contents; the cell-walls 

 forming merely the framework of the workshops in which all 

 the secret and wonderful operations of plants are carried on. 



It is this comparative isolation of an infinite number of 

 vital fragments that constitutes one of the most essential 

 differences between the vegetable and animal series of the 

 organized world. 



7. In our second chapter we spoke of the elements 

 carbon, oxygen, hydrogen, and nitrogen, as existing in plants 

 in a series of peculiar combinations, some of which chemists 

 are not yet able to imitate in their laboratories. These 

 combinations we called ternary and quaternary, (rom their 



