GRASSES OP IOWA. H 



tip to emerge. It points 

 downward and hence is pos- 



_ rr^-J^~ " ^ ^ "~~ — ^ itively geotropic. 



^^Pf^^^r^^P^~^ c:^^^^^^--^^^^^ Harshberger* notes that 



^--x l_,i^=====^L ) " "'■■■ ■■ " before entering the soil, gum 



'^■-Z~- is formed on the tip. We 



V^^^ CD r~~~) have often noticed this, but 

 botanical authors have not 



Fig. 5a. Cross-section through tip of „„^„„„n„ ^^^.^^ ^\^\^ i„4-^_ 



brace root, showing spidermal cells; to the generally noted thlS mtcr- 



left before the addition of water; to the ^„+;»,^ ^i,^v,^vv,^v,« rpu^o, 



right after the addition of water, the outer estmg pnenomena. i niS 



part having expanded. ^j^.^j^ ^^^ -^ ^rOUght about 



by the swelling and later breaking down of the epidermal 

 cell walls. This substance in water takes the form of mucilage, 

 undoubtedly for the purpose of assisting the root to hold itself 

 to the soil. The method of formation of roots has been studied 

 by Harshberger. The developing secondary root shows before 

 the point breaks the epidermis and hypodermis. "Three sup- 

 erimposed hollow cones are found immediately beneath the two 

 outer protective layers, the outer and middle cones being 

 separated by a cushion of parenchyma. The outer cone is 

 composed of actively growing cells with the nucleus and 

 nucleolus plainly visible. It corresponds evidently to the 

 calyptrogen layer of Janczewski. The inner cones corre- 

 spond to the periblem and plerome cylinders with the outer 

 layer of the periblem as the dermatogen or proto-epidermis. 

 The cells of the plerome, destined to form the central vascular 

 system, are much longer than broad, the long axis anticlinal, "f 



Distribution of roots. — It is well known that roots have an 

 important relation to the development of corn, especially in 

 the manner of occurrence. The habit of the roots largely 

 influences the amount of growth the plant can make provided 

 other factors, sunshine and warmth are favorable. 



The development of roots is largely governed by external 

 factors, especially amount of rainfall and moisture in the soil. 



In a paper read before the biological section of the American 

 Association for the Advancement of Science, the following 

 summary was made by the writer: 



" It has been stated that not more than 10 per cent of the 

 roots of corn enter the subsoil. Sturtevant;}; estimates as a 



• Maize, 1. c. 81. 



1 1. c. 81. 



t Conn. Board of Agriculture. 1882: 70. 



