STARCH IN THE LEGUMINOS^ 247 



soon causes a projection upon the exterior of the leucopiast ; 

 it grows considerabh- more or even entirely upon the side 

 which is immersed in the substance of the leucoplast, and 

 the nucleus becomes eccentric and thrown out to the remote 

 side of the leucite." 



What is the mechanism of the growth of the starch grain ? 

 According to M. Dubard, it takes place by apposition, that 

 is to say, by the deposition of new layers upon the surface 

 of the grain already formed; there is no imbibition, that is, 

 no intussusception of particles among those already laid 

 down. 



" The best proof of the apposition is afforded by observa- 

 tion of the cotyledons of Leguminosse while the seeds are 

 being formed. Tissue development being very active the 

 reserves begin to be dissolved; the starch grains are worn 

 away, sometimes perforated ; then, once the tissues of the 

 embryo have become definitely differentiated the storing-up 

 phase recommences and new layers develop around the 

 corroded grain. Whatever the thickness of these new layers, 

 the original granule, of which the appearance has not varied, 

 can always be detected in the centre of the grain. 



Generally all starch grains are characterized bv a series 

 of striations and an initial nucleus or hilum. 



These striations are refractive differences due to an un- 

 equal distribution of water ; the bright layers correspond to 

 the regions which are poor in water, the dark layers to the 

 portions which are richest. When first formed the nucleus 

 is a bright point ; graduallv, as the central portion becomes 

 hydrated, it turns darker, with a bright periphery. This 

 latter growing in its turn the same phenomenon occurs 

 anew. 



According to Payen, the density of starch is from 15 to 

 20. When dried in air at only 20° C. it retains from 18 to 

 35 per cent, of water. It is onl)^ towards a temperature of 

 140° C. that it becomes completely dehydrated. 



Starch is birefractive, a property which is consequent 



