DIV. TI PHYSIOLOGY 227 



this amounts to 5-10 atmospheres, but can sometimes be 100 or more 

 atmospheres. It tends to be unequal even in neighbouring cells, 

 and may show periodic variations in connection with external 

 conditions ( 10 ). 



The separation of the protoplast from the cell wall does not take place so 

 smoothly as shown in Fig. 238. The protoplasm tends to remain connected to the 

 wall by fine strands which rupture later. 



On transference to pure water the turgescent condition will be regained, if 

 the protoplasm has not been injured by the solution. If the protoplasm is killed, 

 however, it has become completely permeable, and the necessary condition for a 

 one-sided pressure has disappeared. Fresh living slices of the Sugar Beet and of 

 the Beet Root when placed in pure water do not allow the colouring matter to 

 escape from the uninjured cells. If the protoplasm is killed, the pigment passes 

 into the surrounding water. 



High osmotic pressures are found in cambium cells (25 atmospheres), nodes of 

 grasses (40 atmospheres), and certain -desert plants (100 atmospheres). The 

 highest pressures are met with in plants, which like those of the sea and sea- 

 shore live in solutions of common salt, or like some Fungi succeed in sugar solu- 

 tions. In these cases also the osmotic pressure of the cell always exceeds that of 

 the surrounding solution ; it is adapted and capable of regulation in relation to 

 the medium, and is therefore not always the same ( n ). It is easy to understand 

 why cells with such high osmotic pressures burst when transferred to less con- 

 centrated solutions or to pure water, in which their turgor pressure is greatly 

 increased. 



The Absorption of Water by more Complex Plants. In many 

 lower plants all the living cells take part in the absorption of water. 

 In more complex plants only the 

 superficial cells are in contact with 

 the supply of water in the environ- 

 ment, and absorption of water is 

 limited to them. In the cormus, 

 at least in the typical land plants, 



the absorption of water is limited Fl( , o 39 ._Tip-of a root-Lair with adhering 

 to the epidermal Cells of the rOOtS. particles of soil, (x circa 240. After NOLL.) 



The suVaerial parts of the plant, 



covered with a more or less strongly-developed cuticle, cannot under 

 natural conditions absorb sufficient water for the needs of the plant. 

 The root, on the other hand, is highly specialised for this purpose, 

 both as regards its external form and the structure of its limiting 

 layer. Since the water in ordinary soils is finely subdivided and held 

 firmly by the particles of the soil, a large surface must be exposed 

 by the absorbing root. This is attained by the extensive branching 

 of the root-system and by the presence of root-hairs which become 

 attached to the finest particles of the soil (Fig. 239). 



The plant is connected to the soil by the numerous lateral roots 

 and their root-hairs, and can thus obtain the water held by capillarity 

 in the soil, as soon as by loss of water a power of suction has arisen 



