VIIL] TISSUES. 95 



Pine-wood (thin slices, both lengthwise and across the 

 "grain"). Long, thick-walled, tapering cells, without 

 any vessels. The sides of the cells are marked with 

 minute disks. 



Thin petals, and petals doubled back to show the pro- 

 jecting cells on the folded edge. 



Pollen. The grains are usually oval or roundish, and 

 generally free from each other when mature. Compare 

 the pollen of Mallow, Cucumber, Fuchsia, Lily, Dead- 

 nettle. 



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 old trunks of trees, the cells forming 

 the older wood lose their active contents, their sides or 

 " walls " undergo molecular change and they cease to take 

 direct part in the life of the tree. 



6. Take some active, sufficiently transparent cells, such 

 as you find upon the margin of a young leaf of Nettle, 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 observe 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 there- 

 fore 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 or individualisation of 

 an infinite number of vital fragments that constitutes the 

 chief structural difference between the vegetable and 

 animal series of the organized world. 



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. The cell-wall consists of carbon, and the 



