26 MINNESOTA, BOTANICAL STUDIES. 



For several years all the internodes are very short and the stem 

 axis elong-ates very slowly (about 4 mm. per year). Its growth is 

 at first upward, but gradually changes into the horizontal growth 

 of the mature rhizome. If the terminal bud is uninjured the main 

 stem does not send out any lateral branches for several years, and 

 the lateral buds which are formed each year remain dormant. 

 Some of them finally grow, producing the characteristic branched 

 rhizome of the mature plant. An injury to the terminal bud will 

 at any time start one of the lateral buds to growth to take the 

 place of the injured terminal. 



The primary root system enlarges for several years. Secondary 

 roots begin to grow out from the older parts of the rhizome after 

 four to ten years' growth, the time depending apparently on the 

 luxuriance of the plant (PI. IX). Several years later the pri- 

 mary root withers and disappears, and thereafter a certain amount 

 of the older part of the rhizome with the attached roots dies each 

 year. Finally when flowering stalks appear (in nature apparently 

 only in very old and well established plants) the stem axis 

 abandons its simple monopodial system of growth, and each year 

 the main axis develops into the subaerial leafy flower-bearing shoot, 

 and the additional growth of the rhizome for the season occurs as 

 a lateral branch from the axil of one of the scale-leaves of the 

 winter bud. 



The Vascular Skeleton of the Seedling. 



Definite dififerentiation of the vascular tissues of the embryo 

 occurs early in the process of germination. The formation of 

 phloem and protoxylem occurs first in the middle bundles of the 

 cotyledons, and shortly afterwards in the lateral bundles. It is 

 well advanced in the lower half of the cotyledons before any dififer- 

 entiation of tissues can be made out in the plerome of the hypocotyl. 

 At the same time the epicotyl still consists of an undifferentiated 

 mound of meristem cells (Fig. 18, PI. V). The dififerentiation of 

 the vascular tissues extends upward into the upper part of the 

 cotyledons, and downward into the hypocotyl. 



Primary Structure of the Hypocotyl. 



The very slow development of the epicotyl together with the 

 strong development of the cotyledons and roots, and the compara- 



