32 ELEMENTS OF PLANT ANATOMY. 
starch-builder, and that whenever the smallest particles of 
starch are formed into an organized structure it is done through 
the medium of this plastid. 
Regarding the origin of the plastids there are two views, 
one, that they are formed by the cytoplasm at any time and 
whenever needed, the other that they increase in numbers only 
by division. Those holding the latter view claim that certain 
structures exist in the fertilized egg cell, such as nucleus and 
leucoplastid, and that as new cells are formed these structures 
divide to supply them, and that the colorless leucoplastids may 
later be impregnated with pigment so as to form both chloro- 
and chromoplastids. 
6. Starch Grains. 
It is customary to divide the food products in plant cells 
into two classes, nitrogenous and non-nitrogenous. ‘The organ- 
ized structures of the latter take the form of starch grains, 
which originate in the chloroplastid by the process of CO, 
assimilation and are changed into sugar before they can be 
transported from the place of origin. Sometimes in the course 
of the passage of sugar from cell to cell, starch occurs as very 
fine grains. In this form it is called transitory, as it changes 
rapidly to sugar and this again to starch. In cells destined to 
hold food for future use, starch is laid up in large grains. It is 
then called reserve starch. It is in the formation of these grains 
that the teucoplastids are supposed to play an active part, 
though it is not proved that they are not sometimes formed 
without the aid of these structures. 
The form of the starch grains is generally roundish, ellipti- 
cal, or egg-shaped. When crowded together in a cell they may 
become polyhedral. In the milk tubes of Euphorbiaceae they 
are club-shaped with enlarged ends. 
The substance of the starch grain is closely related to cel- 
lulose, and has the same formula, C,H,,O;. The grains have - 
