18 COMPENDIUM OF GENERAL BOTANY. 



there appears a bright rectangular cross whose arms form an angle 

 of 45° with the polarization plane of the nicols. 



Of Nageli's arguments in favor of growth by intussusception I 

 Mull now mention the particular one which, according to Schimpek's' 

 more recent investigations, can no longer be maintained. Fig. 7 

 shows two " compound " starch-grains. According to Nageli these are 

 usually the result of differentiations within the starch-grain (usually 

 nuclear division) and only exceptionally through the fusion of 

 two individual grains. Schimper has demonstrated for a large 

 number of plants that subsequent fusion of individual starch- 

 grains does take place. 



The theory of intussusception, whose acceptance is favored by 

 reasons to be given below, teaches that starch-substance in solution 

 (for example glucose), hence starch-molecules and water-molecules, 

 passes into the interior of the growing starch-grain, and that from 

 this material new molecular layers are formed and the size and 

 density of already existing molecular masses are increased.'' The in- 

 crease in density of the starch-substance depends upon the increase 

 in size of the molecules which grow by apposition, similar to crystals. 



The following evidence and considerations speak in favor of the 

 intussusception theory : 1. In the earliest stage the starch-grain con- 

 sists of a uniformly dense substance : such is the nature of the very 

 small grains; however, as soon as they increase in size, there is formed 

 a softer, more watery (not denser) nucleus or central layer. 2. The 

 outermost layer of the growing starch-grain is always more dense. 

 3. The demonstrated presence of the internal tensions of the starch- 

 grain also harmonizes with Niigeli's theory. In the outer layer of 

 the young, still firm, spherical starch-grain there originates and 

 exists a positive tension ; in the interior a negative tension. The 

 outer layer receives the first and greatest food-supply, and as a 

 result it is first to increase in area. When the negative pressure in 

 the interior mass (plus the effort to deposit food-material) has reached 

 a certain height, the soft nucleus (hilum) is formed. Similar 

 processes take place in the outer dense layer ; this is repeated 

 again and again. It may be stated here that Niigeli believes the 

 causes for these molecular changes, namely, the attraction of starch- 

 particles for each other and for watei', to be certain molecular 



» Botaaische Zeitung, 1880, 1881. 

 ''Nageli, Starkekomer, S. 2yi. 



