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PART III. PHYSIOLOGY. 



47 



is afforded by Spirogyra and other filamentous Algae. It is this 

 growth in length which has been more especially studied physio- 

 logically, and in what follows, " growth " may be taken to mean 

 " growth in length," unless there is some definite statement to the 

 contrary. 



The growth in length of the plant-body takes place at first 

 throughout its whole extent ; but at a later period it is limited, as 

 a rule, to particular regions (see p. 8). In the growing portion 

 of any member two regions may be distinguished : the formative 

 region, which is the growing-point proper: and the region of 

 elongation adjacent to it (Fig. 130). In the formative region the 

 construction of the new tissue from plastic substances takes place, 

 as is specially manifested in the formation of cell-walls accom- 

 panying the cell-division going on in this region of a multicellular 

 growing-point; but the amount of elonga- 

 tion is slight. In the region of elongation, 

 the formative processes have ceased : in 

 multicellular plants little or no cell-division 

 takes place in this region ; the cells are 

 here fully formed, and they simply require 

 to increase in bulk, to grow in fact, in 

 order to attain the mature form. Beyond 

 the region of elongation comes the portion 

 of the member which has already ceased to 

 grow. It must be clearly understood that 

 each portion of the growing-point passes 

 through these three phases. For instance, 

 in a multicellular apical growing-point, 

 each cell is produced in the formative re- 

 gion ; and as in consequence of the con- 

 tinued formation of younger cells in front 

 of it at the apex, it comes to lie at an 

 increasing distance from the apex, it passes 

 through the stage of growth, to become an 

 adult tissue-element. 



The movement of growth in length is 

 altogether spontaneous. It may be generally described as the 

 travelling of the organic apex in a line which is the continuation 

 of the longitudinal axis of the growing member. Both the rate 

 and the direction of growth are liable to variation. 



Variations in the Rate of Growth. When a member begins to 



KIG. 130. The growing 

 primary root of the Pea in 

 two stages. .4 The root is 

 marked by lines atequal dis- 

 tances. In B the differences 

 in rapidity of growth are 

 perceptible : the uppermost 

 lines have not been sepa- 

 rated ; the root has ceased 

 to grow here. The lowest 

 likewise are still close toge- 

 ther ; at the apex elonga- 

 tion has not taken place. 

 In the intermediate zones 

 the elongation has been 

 very great. 



