424 STORAGE SYSTEM 



28 per cent., whereas the width (in the direction at right angles 

 to a-b) will be augmented to the extent of 11 per cent, at most. 

 The orientation of the two domes is thus closely correlated with the 

 direction in which the surrounding soil exerts the greatest pressure 

 when the seed swells under natural conditions. It may also be seen 

 from the figure, that the lines of curvature of the radial cell-series 

 indicate the paths along which the mobilised reserve-materials are con- 

 veyed to the embryo during germination. It is self-evident that this 

 arrangement of the cells in converging rows must considerably 

 accelerate the depletion of the storage-tissue. Evidently Ancthum 

 illustrates the case in which a single system of curved cell-series is 

 concerned both with the mechanical and with the nutritive require- 

 ments of the organ in question. 



The seed of Iris may be selected as an example of a different 

 type of arrangement. In Fig. 164 A this seed is shown in tangential 

 section. The pointed lower end of the seed contains the embryo (e) ; 

 in the upper truncated portion the elongated and very thick-walled 

 endosperm ceils are arranged in curved series which trend obliquely 

 outwards and downwards. The lines of curvature intersect the paths 

 followed by the mobilised reserve-materials on their way to the 

 embryo at various angles ; they cannot, therefore, stand in any 

 close relation to the nutritive requirements of the embryo, but 

 must, on the contrary, have a purely mechanical significance. A 

 transverse section across the narrow portion of the seed (cf. Eig. 164 b, 

 a-b) reveals the fact that the endosperm cells in this region are dis- 

 posed in straight rows radiating in every direction from the centrally 

 situated embryo, and meeting the surface at all sorts of angles. Here 

 there is clearly no mechanical adaptation. In order to produce in- 

 creased resistance to radial pressure, the lines of curvature should form 

 two dome-like structures placed back to back, each resting upon 

 one of the flattened surfaces of the seed ; as already stated, however, 

 the radial cell-rows are perfectly straight. From the point of view of 

 translocation, on the other hand, the arrangement of the endosperm- 

 cells in the neighbourhood of the embryo is highly appropriate, since it 

 brings every part of the storage-tissue into the closest possible relation 

 with the embryo. The endosperm-cells of Iris are thus grouped 

 in series belonging to two totally distinct systems ; one of these, 

 which can be made out in a tangential longitudinal section of the seed, 

 is of purely mechanical significance, while the other, which is only 

 visible in a transverse section, is developed in the interests of nutrition. 



In many seeds, the cells of the endosperm are elongated at right 

 angles to the surface, so as to resemble palisade-cells in form. Thus, 

 a transverse section through the seed of Polygonatum valgare 



