600 



SENSORY SYSTEM 



displaced from its stable position ; in order to achieve the same result 

 in a greatly elongated cell, the starch-grains would have to form a very 

 thick stratum in the stable position, in Fumaria officinalis the sensory 

 cells are arranged in longitudinal rows composed of alternating long and 

 short elements ; the latter are one and a half to three times wider than 

 their length, and thus appear more or less disc-shaped. In transverse 

 section the elements of the starch-sheath are usually four- to six-sided 

 and tangentially elongated ; more rarely they are nearly circular in out- 

 line (Vinca minor, Fig. 251 A). 



The starch-grains which act as statoliths, are sometimes simple and 

 spherical, but more often belong to the compound type, and consist 

 of two or more partial grains. As a rule they are considerably 

 larger than the ordinary starch-grains of the medullary and cortical 



A 



Fig. 251. 



A, Small portion of a T.S. through a stem of Vinca minor, which has heen lj'ing 

 horizontally. B. Small portion of a T.S. through epiootyl of Phaseolns multiflorus ; the 

 portion figured has been on one of the flanks, when the stem was lying horizontally. 



parenchyma. They are generally enclosed within pale-green chloro- 

 plasts, or more rarely within amyloplasts ; in either case the stroma of 

 the plastid persists as a thin pellicle around the statolith. The number 

 of statoliths contained in each sensory cell varies much in the same 

 way as it does in the case of roots ; in the erect position of the stem 

 the starch-grains most often form a single layer on the lower transverse 

 wall of each statocyst, but two or three layers are found in certain cases. 

 According to Schwaighofer, the sensory cells in the hypocotyls of 307 

 certain Compositae contain abnormally small numbers of statoliths. 

 In Madia sativa, for example, each statocyst usually contains five starch- 

 grains ; 5*6 per cent, of the statocysts examined actually contained 

 only a single starch-grain, which responded very readily to the influence 

 of gravity. Cases of this kind militate strongly against the theory which 

 regards falling starch as a form of reserve-material. 



