33^ 



LECTURE XX. 



this is the case, for example, in the endosperm of the Oat, of Mirabilis Jcilappa, and 

 other seeds. Naegeli terms such forms as those in Fig. 234 B, C, and D semi-com- 

 pound : in these cases two or three starch-grains lie not only with their surfaces 

 closely appressed to one another, but they are also embraced by common en- 

 veloping layers. 



According to Schimper's more recent observations, compound starch-grains 

 arise as follows. In the interior of a starch-forming corpuscle are formed starch- 

 grains, at first like mere dots, then increas- 

 ing in size : these finally come into contact 

 with one another, and flatten, the surface 

 of the whole mass, corresponding to the 

 circumference of the protoplasmic starch- 

 forming corpuscle, remaining rounded — a 

 process which also takes place in the form- 

 ation of starch in chlorophyll-corpuscles (cf. 

 Fig. 222). As regards the semi-compound 

 starch-grains, it results from Schimper's 

 description that the grains are produced at 

 two, three, or more points of the surface 

 of a starch-forming corpuscle, and grow 

 most vigorously at those surfaces which are 

 in contact with the starch-forming cor- 

 puscle, whence the hilum of each comes 

 to lie eccentrically and towards the outside ; 

 and when the substance of the starch- 

 forming corpuscle itself is at length com- 

 pletely consumed, the partial grains are 



mutually in contact at their surfaces of 



^y W°J!;'f ^ m 1 growth. All simple starch-grains with a 



central hilum and concentric structure 

 originate, according to Schimper, entirely 

 in the interior of a starch-forming cor- 

 puscle ; and are therefore enveloped by its 

 substance, from which they are nourished. 

 The eccentrically constructed grains, on 

 the other hand, arise beneath the surface 

 of a starch-forming corpuscle, and break 

 through this as they become larger ; 

 they then grow at the side connected 

 with the starch-forming corpuscle, so that the hilum is pushed outwards eccen- 

 trically. 



To the best known peculiarities of starch-grains belongs their stratification. 

 Around the hilum already mentioned the substance of a starch-grain is not homo- 

 geneous, but consists of alternating layers of different density, surrounding the hilum 

 concentrically from within outwards : in grains with eccentric structure, only a cer- 

 tain number of such closed layers envelope the hilum, while more or less numerous 



Fig. 235.—^ a cell from the endosperm of Zea Mais, filled 

 with closely packed and therefore polyhedral starch-grains ; 

 between the grains are thin plates of dry fine-grained proto- 

 plasm. Small cavities and fissures have arisen in the grains 

 owing to the drying. A^£ starch-grains from the endosperm 

 of a germinating seed of Maize. />' lenticular starch-grains 

 from the endosperm of a germinating seed of Triticnrn vitl^ 

 gare : the commencing action of the ferment makes itself 

 evident in the first place by the stratification becoming more 



