(;kk\ti-v em.ak<;ki) ckoss sections of iviacarom and soft wiieat^^ 



Cross sections of wheat grains ])hotograiJhcd In' transmitted light, (a) Pure macaroni. Endo- 

 sperm texture flinty and translucent. Consequently photographing white, (h) Pure Sonora. 

 Endosperm texture soft, rendered nearly opaque by many small difTusely scattered air spaces, 

 which photograph dark. (Fig. 12.) 



those crosses in which tlie niucaroni 

 wheat furnislied one of the parents. 



The (h (Terence hetween the texture 

 of the hard, transhicent iijrains of maca- 

 roni wheats and the opafjue grains of 

 soft wheats lies in the proportion 

 of ghiten to starch and tlie hehavior of 

 these compounds in the ripening of 

 the j^rain. When sufficient ,t,duten is 

 present the entire cell contents, includ- 

 inj^ the starch t^rains. remain cemented 

 toj^a-ther. as the ^.jrain dries out in rii)en- 

 ing. and the decrease in wei<(ht occa- 

 sioned hy the loss of water is accom- 

 panied hy a corrcsi)ondin«( shrinkage 

 in the size oi the grain. The solidly ce- 

 mented mass therefore hecomes translu- 

 cent and glassy in texture as the grain 

 approaches the air dry cnndition. ( )n 

 the other hand, when insulTicient gluten 

 is present, the starch grains are not 

 held lirmly together aiul when the grain 

 dries out. instead of shrinking to fully 

 compensate for the loss of water, air 

 spaces appear. These air spaces si-rve 

 as refracting surfaces and render the 

 grain opaf|ue. 



In order io in.ake a niicrciscopjcal 



212 



study of grain texture, it is necessary 

 to obtain thin sections without disturb- 

 ing the physical structtire of the grain. 

 'Jhe brittleness of hard wheats and the 

 friability of those which are soft make 

 it impracticable to cut thin sections 

 with a razor from the dry grain. On 

 the other hand, if the seed he soaked 

 uj) witli water or other softening solu- 

 tions ilu- physical structure of the 

 endosperm is disturbed and does not 

 return completely to its former con- 

 dition when subse(|uently desiccated. 

 I'^ortunately these dititicullies may be 

 overcome by making sections after the 

 method used by petrologists in pre- 

 paring thin sections of mineral or fos- 

 sils for microscopical sttidy. A section 

 about 2 mm. thick is easily cut. or 

 rather broken out. with a knife. F>y 

 rubbing this section over tlie surface 

 of a line, flat tile it may be (|uickly 

 ground down to the thickness of ordi- 

 tiarv writing paper. It is then trans- 

 ferred to the clean, dry snrf.icr ol a 

 vellow I'.elgian hone upon wliich it is 

 furtlu'r i)olishe(l on both sides. Ibis 



