SECT. II 



PHYSIOLOGY 257 



By the growth in surface of the cell wall the volume of the cell may 

 increase a hundredfold or more without any marked increase in the 

 amount of protoplasm. The enlargement of the cell has been almost 

 wholly produced by the increased volume of water in the sap cavity, 

 which (to distinguish it from the " imbibition water," and " constitution 

 water" of the plant) may be designated "distension water." 



As is observed in a whole series of vital phenomena, the rate 

 of distension of the walls with the distension water is not uniform, 



but BEGINS SLOWLY, INCREASES TO A MAXIMUM RAPIDITY, AND 

 THEN, GRADUALLY DIMINISHING, CEASES ALTOGETHER. This GRAND 



PERIOD of growth appears in its typical form when all external 

 conditions are constant. It is true that occasionally very sudden 

 changes in the rate of growth occur, due to internal and as yet 

 unknown causes, which disturb the regularity of the grand period of 

 growth. 



The process of elongation has so far been considered only in 

 relation to the single cell, preparatory to the consideration of the 

 phenomena presented by the growth of multicellular organs. This 

 proceeds so slowly as to be, in genei'al, imperceptible. Only 

 some fungal hyphae and the stamens of some Gramineae grow so 

 rapidly that their elongation is evident, even to the naked eye. 

 The fructification of Didyoplwra grows in length to the extent of 5 mm. 

 per minute (A. Moller), and according to Askenasy an increase in 

 length of 1"8 mm. a minute has been observed in the stamens of 

 Tritimm (Wheat). This approximately corresponds to the rate .of 

 movement of the minute-hand of a watch. In compainson with these 

 the next most rapidly growing organ known is the leaf-shcath of the 

 Banana which shows an elongation of 1"1 mm., and a Bamboo shoot, 

 an increase in length of 0*75 mm. per minute ; a strong shoot of 

 Cucurbita grows O'l mm. per minute, the hyphae of BotryUs grow 

 0'034 mm., while most other plants, even under favourable circum- 

 stances, attain but a small rate of elongation (0'005 mm. and less per 

 minute). 



In ordei' to measure tlic growth in length of a plant, it is customary to magnify 

 in some way the actual elongation for more convenient observation. This may be 

 eft'ected by means of a microscope, which magnifies the rate of growth corre- 

 spondingly with the distance grown. For large objects, the most convenient and 

 usual method of determining the rate of growth is by means of an auxanometeii. 

 The principle of all auxanometers, however they may differ in construction, is the 

 same, and is based upon the magnification of the rate of growth liy means of a 

 lever witli a long and short arm. In Fig. 209, at the left, a simple form of auxano- 

 meter is shown. The thread fastened to the top of the plant to be observed 

 is passed over the movable pulley (r), and held taut by the weight {(j), which 

 should not be so heavy as to exert any strain nn the plant. To the pulley there 

 is attached a slender pointer (~), which is twenty times as long as the radius 

 of the pulley, and tliis indicates on the scale (S) the rapiditj' nf the growth 

 niagnilied twenty-fold. 



S 



