r,2 



MINUTE STRUCTURE. 



every plant at some time or other contains tliis substance, 

 and wlien at length it disappears, the cells which are 

 deprived of it no longer take any active part in the growth 

 of the plant, but serve merely mechanical purposes, such 

 as that of support or conduction, and are, in that stage of 

 their historj', filled usually with air or water. The pith 

 of the Elder is made up of such dead cells, as is also 

 the greater part of the wood and bark and older parts 

 generally of all plants. 



The most marked feature of the living protoplasm is its 

 (ivtivity. We may observe this property' by exainining 

 plant-hairs and other parts under high powers of the 

 microscope, when it will be seen that there are move- 

 ments of two kinds. The whole mass of protoplasm has 

 a rotary motion, sliding upon the cell-wall, downwards 

 on one side and upwards on the other. This is the 

 'mass-movement. Also currents may be traced passing 

 across the protoplasm in different directions. This is the 

 streaming-movement. In Fig. 258 the arrows show the 

 direction of the currents. 



In some of the ver3' lowest plants, where there is no cell-wall, 

 and the whole is- a mass of naked protoplasm, these 

 movements may be observed more readilj' because they are 

 less restricted. 



There is some doubt as to the exact chemical composition of 

 protoplasm. It is, however, a verj' complex substance 

 belonging to a group of bodies known as albuminoids, of 

 which nitrogen is an important constituent. 



The consistence of protoplasm depends upon the amount of 

 Avater it contains. In dry seeds, for example, it is tough 

 and hard, but Avhen the same seeds are soaked in Avater 

 it becomes i^artially liquid. 



Forms of Cells. 



As cells become older they tend as a rule to change their 

 form, though sometimes we find them differing but little 

 from their original conformation. Commonly a cell 

 grows more rapidly in some one direction, thus giving 

 rise to long forms, as is the case in stems generally, and 

 in the i^etioles and veins of leaves, the superior toughness 

 and strength of which are due to the lengthening and 

 hardening of the cells of which they are composed (Fig. 

 259). 



The Cell-wall. 



In the i)ortions of plants just selected for microscopic exami- 

 nation we have seen that tlie protoplasm is in every 

 instance bounded by a wall. It has been ascertained 

 that the wall is a chemical compound of carbon, hydro- 



FiR. 257. 



Ofjp 







Fig. 259. 



