22 BOTANY OF THE LIVING PLANT 



are seen, till the mature parts are reached. Various changes appear 

 in the cells. They alter their form and the character of their walls 

 and contents. This leads to differentiation of the several types of 

 tissue which compose the mature parts of the shoot. As a rule the 

 cells enlarge greatly. An important change in the cytoplasm, which 

 is usual in plant-cells, accompanies this growth. It is known as 

 vacuolisation, and it may be well illustrated in the cells developing into 

 the pith or cortex of an ordinary stem. Starting from the embryonic 

 state, where the wall is very thin, and the cytoplasm and nucleus 

 fill the whole space enclosed by it (Fig. 12, A ), the volume begins to 

 increase with age and the wall thickens. But the volume of the 

 cytoplasm does not keep pace with that of the whole cell, and vesicles 

 or drops of clear fluid appear within it. These are called vacuoles^ 

 and they are filled with vacuole fluid, or cell-sap^ which is water 

 with certain substances dissolved in it (Fig. 12, B). The vacuoles 

 are always completely enclosed in the cytoplasm, which controls them 

 and the substances dissolved in them. The vacuoles may vary in 

 number, size and position, and the position of the nucleus is also 

 inconstant ; sometimes it lies laterally in the peripheral cytoplasm ; 

 usually it is central. As the vacuoles enlarge they may run together, 

 and finally form a continuous cavity in the middle of which the 

 nucleus is frequently suspended by radiating threads of cytoplasm 

 (Fig. 12, C). A condition is thus arrived at which is characteristic 

 of many cells in the mature state. 



The vacuolated condition of the cell may be more conveniently 

 studied in the living state in cells forming the hairs, which project 

 from the surface of the plant, than in those forming a solid tissue. 

 Such hairs can easily be removed and mounted living in water (Fig. 13). 

 Careful observation will then show that the cytoplasm is not quiescent, 

 but during life it carries out more or less active movements. Granules 

 embedded in the cytoplasm may be seen gliding back and forth along 

 the threads that suspend the nucleus, or along that peripheral film of 

 cytoplasm which in living cells always completely lines the w^all 

 internally. The rapidity of these circulating movements may be 

 measured by the time a granule takes to traverse one unit of a micro- 

 meter scale. Experiments at varying temperatures show that the 

 rapidity depends upon temperature. There is an optimum tempera- 

 ture at which the movement is most rapid, and maximum and minimum 

 temperatures at which it ceases. Anaesthetics and the absence of 

 oxygen stop it. Exposure to the temperature of boiling water, or 

 to poisons such as alcohol, also stops the movement, for these 



