BOTANY 



depressions (Fig. 29) or tubular canals (Fig. 30), closed at one end, as 

 a rule the outer, by the unthickened portion of the cell wall which 

 forms the pit membrane (Fig. 29 J5). Sometimes with the increase 

 in the thickness of the wall the canals of several pits unite forming 

 BRANCHED PITS. Such branched pits have usually very narrow canals 

 and occur for the most part in extremely thick and hard cell walls as, 

 - [B|I IP _ j_q . ^ or ^stance, those of sclerotic 



'"f ' f ,jBl-. JJIIII IJ^ , ^k ^ gc j ere - c j eg pj^ gQ^ 



^z-^^^^^^^ In other colls the greater part 



FIG. 31.-Portion of a tubular rhizoid of Marchantia with f the wal1 is Only slightly 



local peg-like thickenings of the wall. thickened, while narrowly 



circumscribed portions 



thicken greatly and assume the form of projections, warts, simple or 

 branched pegs (Fig. 31), spines (Fig. 32), ridges, bands or a network 

 (Figs. 67, 68). Such thickenings may form either on the outside 

 (centrifugal) or on the inner surface of the wall (centripetal). Small 

 projections often occur on hairs, while the thickenings of spores 

 and pollen grains (Fig. 32) and in many water-conducting cells of 

 the higher plants (Figs. 67, 68) are characteristic. 



A very peculiar form of thickening with calcium carbonate deposited in it and 

 localised to one small region of the wall is seen in the CYSTOLITH which forms a 

 stalked body, hanging in the cell like a bunch of grapes (Ficus elastica, Fig. 33). 



The growth in thickness, which commences during the growth in 

 surface of the wall, continues after this is complete. It is usually 

 effected by apposition, i.e. the 

 deposition of material by the 

 protoplasm on the already exist- 

 ing wall in the form of new 

 layers or lamellae. In this way 

 a concentric stratification of the 

 cell wall arises (Fig. 30). In 

 the thickened wall thicker, denser 

 lamellae alternate with thinner 

 and less dense layers, which are 

 often not only richer in water but 



A 



chemically different from the FlG - 32 - -4, Pollen-grain of CucurUtaPepo in surface 

 , rpi -i , view, and partly in optical section, rendered 



denser layers, Ine latter are transparen t by treating with oil of lemons. 

 more highly refractive and appear ( x 240.) B, Part of transverse section of pollen 

 brighter. In many, apparently s rain of CucurUta verrwosa. (x 540. After 



11 vi i STRASBURGER.) 



homogeneous, cell walls such 



stratification can be recognised after swelling has been brought about 



by treatment with strong acids or alkalies. 



Not uncommonly growth in thickness also depends on the 

 introduction of new material into the existing wall (intussusception). 



Centrifugal thickening of the wall is frequently brought about by intussusception. 

 This can take place at some distance from the protoplasm and be associated with 



