54 DIFFERENTIATION AND SPECIFICITY' OF STARCHES. 



of a growing crystal, the part already deposited influences the shape of the subsequent 

 additions. The internal strains observed in starch-grains, he holds, could easily be pro- 

 duced by apposition combined wdth subsequent internal changes, and no arguments as 

 to the mode of growth can be deduced from the supposed molecular structure, which is 

 itself a mere hypothetical abstraction. The power of forming starch is, according to Pfeffer, 

 possessed by etiolated chloroplasts as well as by many non-chlorophyllous chromatophores, 

 but all chromatophores have not this power, and certain chloroplastids never contain 

 starch, perhaps because solvent enzymes may be present which dissolve the starch as fast 

 as it is formed. 



The sequence of events which occur in the formation of the starch-grain was studied 

 by Timberlake (Annals of Botany, 1901, xv, 619). His inquiries were made with Hydro- 

 dictyon. He notes that the pyrenoid (first described by Schmitz) is a spherical protein 

 body that forms a part of the chromatophore, and that it bears a morphological relation 

 to it similar to that of the nucleolus to the nucleus. In cells of this plant chlorophj'l is 

 distributed in the whole peripheral protoplasmic layer of the cell. No distinct chromato- 

 phores are observable in the cells. In the cells that contain an abundance of starch, prac- 

 tically the whole layer of the protoplasm, from the plasma membrane on the outside to 

 the vacuolar membrane on the inside, is filled with starch-grains, all of which originate in 

 pyrenoids and later are transferred bodily to other parts of the cell. The whole process of 

 starch formation could be traced from certain structural changes occurring in the body of 

 the pyrenoid. 



The first indication of the changes leading to the formation of starch consists, Timber- 

 lake found, in a difTerentiation of the body of the pyrenoid into two portions, one of which 

 is destined to become transformed into a starch-grain and the other to remain unchanged. 

 The part that is to form the starch-grain stains less densely, and instead of red becomes a 

 neutral gray or a faint orange, in safranin-gentian-orange stain. The dense homogeneous 

 structure becomes spongy, with regions of varying density. Very often the dense regions 

 are so distributed as to give an alveolar appearance. The denser regions become more 

 prominent and take up the blue stain. Between the fully formed starch-grains and the 

 unchanged remainder of the pyrenoid a thin zone of slightly stained material appears. 



When the grain is fully formed it is seen to he in a vesicle or vacuole in the cytoplasm, 

 but without being surrounded by a differentiated membrane. The mature grain was 

 observed to have practically the shape of the pyrenoid from which it was formed. This 

 point, states Timberlake, aids in establishing that all starch is formed from the pyrenoid. 

 When starch is produced rapidly a second grain will be built at once before the pyrenoid 

 regains its original form. The long axis of the second grain is at right angles to that of 

 the first. As the process precedes rapidly, the grains as they are formed are continually 

 crowded outward by the last-formed grains, until finally they are densely packed through- 

 out nearly the whole of the protoplast. He therefore looks upon the pyrenoid as being 

 directly the seat of a process which results in the formation of starch, which process, he 

 states, is an exceedingly complicated one, as is shown by the structural and microchemical 

 changes ehcited by differences in staining. 



Since the pyrenoid seems to be of a protein nature, and since a part of it (at least in 

 Hydrodictyon) seems to be converted into starch, Timberlake suggests that the process 

 involves the breaking down of a protein into carbohydrate. He notes that Boubier sug- 

 gested the hypothesis that the pyrenoid is comparable to the leucoplast of the higher 

 plants, and that the method of starch formation in it is similar to that in the leucoplast. 

 The most serious objection to the comparison suggested by Boubier seems to lie, Timber- 

 lake believes, m the fact of the difference in structure between the two. Timberlake 

 states that it is difficult to differentiate the leucoplast from the rest of the protoplasm, and 

 that when it is differentiated it has a granular or reticulate appearance; but the pyrenoid 



