382 



VEGETABLE GROWTH. 



1. Periclinal, those which exhibit in longitudinal section 

 curves in the same direction as the surface. 



2. Anticlinal, those which cut the surface and the periclinal 

 walls at right angles (forming a system of orthogonal trajecto- 

 ries for the periclinal walls). 



3. Radial, those which pass through the axis of growth and 

 cut the surface at right angles. 



4. Transverse, those which cut both the axis of growth and 

 the surface at right angles. 



1003. Growth of the cell- wall. When the new cell is formed 

 it undergoes changes in size, and often in shape and thickness. 

 If it increases in size regularly at all points of the surface, it 



preserves, of course, its 

 original shape ; but if its 

 growth is irregular at 

 different points, great 

 modifications of form re- 

 sult. Pollen-grains afford 

 instances of the former 

 method of growth, while 

 the latter is seen in the 

 multicellular organs, for 

 example stems and leaves. 

 At the growing points of 

 the stem and leaf the 

 cells when first formed are 

 nearly alike in appear- 

 ance ; but wide differ- 

 ences are soon presented. 

 The growth of a cell 

 in size may be terminal, 

 when it gives rise to 

 elongated forms ; or lo- 

 calized at a point, line, 

 or zone, when projections 

 and swellings of various kinds are produced. 1 



Arbeiten des botan. Inst. in Wiirzburg, 1878, in appendix to Text-book, 2d 

 Eng. ed., p. 951). 



The student should also read Sachs's Vorlesungen, 1882, pp. 523-557. 



1 These have already been sufficiently considered in the histological part 

 of this volume, and it is not necessary to again call attention to the adaptations 

 of the resultant structures to their respective kinds of work in the organism. 



FIG. 168. Arc-auxanometer /, thread connecting plant with short arm of lever a z. 

 The weight of long arm balanced by movable weight at k. ( Pfeffer.) 



