9 PHYSIO LOG Y. 



soaked in water for 24 hours and note the form and difference in the two 



sides (in all of these studies the form and structure of the seed, as well a;- 



the stages in germination, should be illustrated by the student). Make a 



longisection of a grain of corn through the middle line, if necessary 



making several in order to obtain one which shows the structures well near 



the smaller end of the grain. Note the following structures: ist, the hard 



^^^ outer ' ' wall' ' (formed of the consoli- 



/ r ~' x \^*^ -i : V_ f \^^^ dated wall of the ovary with the in- 



V^ ^^^* \^2A^ teguments of the ovules see Chap- 



ters 35 and 36) ; 2d, the greater mass 



Section of com seed' 'at upper right of of starch and other P lant food (the 

 each is the plantlet, next the cotyledon, at endosperm) in the centre: ?d, a some- 

 left the endosperm. ^ 



what crescent-shaped body (the 



scutellum) lying next the endosperm and near the smaller end of the 

 grain; 4th, the remaining portion of the young embryo lying between the 

 scutellum and the seed coat in the depression. When good sections are 

 made one can make out the radicle at the smaller end of the seed, and a 

 few successive leaves (the plumule) which lie at the opposite end of the 

 embryo shown by sharply cuived parallel lines. Observe the attachment 

 of the scutellum to the caulicle at the point of junction of the plumule and 

 the radicle. The scutellum is a part of the embryo and represents a coty- 

 ledon. The endosperm is also called albumen, and such a seed is albumin- 

 ous. 



Dissect out an embryo from another seed, and compare with that seen in 

 the section. 



205. In the germination of the grain of corn the endosperm supplies the 

 food for the growth of the embryo until the roots are well established in 

 the soil and the leaves have become expanded and green, in which stage 

 the plant has become able to obtain its food from the soil and air and live 

 independently. The starch in the endosperm cannot of course be used for 

 food by the embryo in the form of starch. It is first converted into a solu- 

 ble form and then absorbed through the surface of the scutellum or coty- 

 ledon and carried to all parts of the embryo. An enzyme developed by the 

 embryo acts upon the starch, converting it into a form of sugar which is in 

 solution and can thus be absorbed. This enzyme is one of the so-called 

 diastatic " ferments " which are formed during the germination of all seeds 

 which contain rood stored in the form of starch. In some seedlings, 

 this diastase formed is developed in much greater abundance than in 

 others, for example, in barley. Examine grains of corn still attached 

 to seedlings several weeks old and note that a large part of their content 

 has been used up. The action of diastase on starch is described in 

 Chapter 8. 



