Mar. is, 1934 
Studies on the Potato Tuber 
823 
certain structures differentiate out, which, because of their characteristic 
appearance, have recently been the cause of some misconception as to 
their nature and function. This peculiar type of sieve tube inclusion 
was first observed by Nelson ( 10) in the potato and other plants. The 
bodies were described as straight or twisted rods with pointed ends and 
occasionally bearing an attenuate process at one or both ends. The 
true value of the discovery was temporarily marred by an attempt to 
identify these bodies with certain flagellates and ascribing to them a 
probable r 61 e in the production of the mosaic disease. However, 
although these bodies were wrongly identified and their supposed con¬ 
nection with the mosaic disease was further invalidated by the fact of 
their occurence in healthy as well as in diseased plants, their discovery 
is not without interest. An insight into their origin and nature was 
much to be desired, and some attention has been given to them in this 
morphological study of the potato tuber. 
The bodies first become noticeable as light staining, looped or undulat¬ 
ing bands along the inner margin of the cell or, more commonly, adhering 
to one side (PI. 7, G). Often the bands are short, pointed and somewhat 
spirally twisted (PI. 7, E, H; PL 6, C, B, E) abutting on the nucleus of 
the cell (PI. 7, G) or lying next the end wall (PI. 6, A, B, E). The 
bodies exhibit an extreme polymorphism, which is very pronounced 
within a single element (PI. 7, F). Once formed they appear to retain 
their shape and change only in staining quality. They show at an early 
stage a strong affinity for acid fucfisin (PL 1, D) and safranin. When 
carefully stained with Haidenhains haematoxylin, they present dis¬ 
tinctly a dark staining outer shell and a lighter interior (Pl. 6, D), in 
which are embedded dark granules. Old bodies may, however, stain 
uniformly black. 
These structures usually originate in the cytoplasm of the sieve tube, 
yet in some cases their nuclear origin appears indisputable. They are 
abundant in the sieve tubes of the growing apex, but are also found 
in more mature tissue where new phloem groups are developing; they 
are typically a constituent of the young element, since they are not 
observed in cells with a well-developed sieve plate. Their elective 
power for acid fuchsin and their positive reaction in certain microchemical 
tests (Biuret, Millon) show them to be protein in nature. They are 
not typical protein crystals, but rather bodies homologous to those 
described by Strasburger (18, p . 196, PL 3) and Mrazek (9) for the 
Papillionaceae. They are undoubtedly concerned in the metabolism 
of the sieve tube, but their real significance leaves room for much specu¬ 
lation for which only comparative studies on an extensive scale can give 
a proper basis. 
CHEMICAL CONSTITUTION OF THE TUBER 
Organic Storage Products. —The most important reserve material 
of the tuber is starch, which is deposited soon after the stolon tip swells 
to form the tuber. The first starch is formed in the cells near the vascu¬ 
lar tissue, commonly in the form of fine round grains. Somewhat later 
starch becomes noticeable in the cells of the cortex, and finally in the 
pith. The typical grains are ovate, about 40 to 100 microns long. In 
cross section the grains are spherical. While the small starch grains are 
more or less homogeneous, the large oval grains have an excentric hilum 
and show distinct striation. Compound starch grains are occasionally 
