STARCH GRAINS. 



59 



appearance, are extremely common in the parenchyma of quickly growing plants {e.g. 

 seedlings of Phaseolus, stem of Cucurbita). Grains of this description are different 

 in their origin from compound grains of the kind which occur in chlorophyll ; in this 

 latter case a number of small grains exist in the first place, w^hich only touch and 

 adhere to one another in consequence of increase of size. (Cf. Fig. 47, p. 47.) 



Partially compound starch-grains result when new nuclei and surrounding 

 masses of layers are formed in one grain after each one has already formed 

 several layers. The secondary grains appear therefore to be imbedded in the mass 

 of- layers of the mother-grain. In this case also tension arises from the unequal 

 growth of the common layers and of those belonging to each secondary grain, 

 leading at length to the formation of fissures ; but these do not usually extend to 

 the outside ; the secondary grains remain united. 



(a) T^he Groivth of grains of starch by intussusception must be inferred from the 

 following considerations : — Supposing that the formation of layers occurs from without 

 by deposition, grains would be found the outermost layer of which would be a watery 

 one ; this, however, never occurs ; the outer- 

 most layer is always the densest and least 

 watery. According to this supposition the 

 nucleus would also possess the properties of 

 the youngest grains, M'hereas the nucleus is 

 always soft, the youngest grains dense. The 

 theory of apposition could only be brought 

 in to explain the formation of the partially 

 compound grains ; if we were to suppose that 

 the common layers of a grain which is forming 

 secondary grains had been subsequently de- 

 posited around two or more previously iso- 

 lated grains, then the common layers would 

 have a different form, and the fissures in the 

 interior of such grains remain unexplained. 

 The theory of apposition, finally, is incom- 

 petent to explain why, in secondary grains, 

 the strongest growth always takes place in 

 the line of union of their nuclei (Fig. 51). 

 The possible hypothesis of a deposition of 

 new layers from within would presuppose that 

 the starch-grains were at least temporarily 

 hollow bladders, w^hich has never been ob- 

 served ; on this hypothesis, moreover, it can- 

 not be explained how the phenomena arise 

 which occur in the formation of 'half and 

 secondary grains ; and the only hypothesis 

 which can be accepted is growth by intussus- 

 ception, namely in the direction of the sur- 

 faces of the layers. The hypothesis of the 



growth of starch-grains by intussusception alone affords the simplest explanation of 

 all phenomena ; and, after Nageli's researches, may be considered as a fully 

 established fact. The formative material which penetrates from without into the grain 

 once formed and there becomes deposited in the form of new particles of starch, 

 is, of course, in solution ; but its chemical nature is not yet certainly known ; dissolved 

 starch can never be found to exist in the plant, at least in those cells where active 

 formation and growth of starch-grains has been observed. It is, however, probable that 



Fig. 51.— starch-grains from the tuber of a potato (xSoo). 

 A an older simple grain; B a partially compound grain; 

 C, D perfectly compound grains; E an older grain, the 

 nucleus of which has divided ; a a very young grain, b an 

 older grain ; c a still older grain with divided nucleus. 



