3IO THE STRUCTURE OF ECONOMIC PLANTS 



GENERAL MORPHOLOGY 



The alfalfa plant is a long-lived perennial, its average length 

 of life being from five to seven years, depending upon environmental 

 conditions. It is know^n as a deep feeder and grows best in soils 

 of a type in v^hich a deeply penetrating root system can develop. 

 Where this is not possible, there is an increase in the amount of 

 branching and in the lateral spread of the branches formed. 



The Root and Shoot. — The young plant develops a tap root 

 that penetrates very deeply and rapidly, often reaching depths 

 of five to six feet the first season of growth. (Fig. 155) Garver 

 (ii) estimated the volume of soil occupied by a single three-year- 

 old plant to be 3 feet in diameter and 10 feet in depth. According 

 to Weaver (33), it may penetrate to 10 or ii feet by the end of the 

 second year; and, ultimately, may extend to depths of xo feet or 

 more. It develops a few branches in the first inches of surface soil; 

 but these, instead of spreading laterally, penetrate more deeply 

 and finally follow a course parallel to the tap root. Frequently, 

 branches are limited in number and the tap root is always the most 

 important part of the root system. 



From the short stout crown stem, numerous erect branches 

 arise which attain a height of i to 4 feet. In hardy types, such 

 as Grimm alfalfa, the woody rhizome is branched, and shoots arise 

 from it below or at the soil surface forming a bushy crown. The 

 upper portions are herbaceous; and the slender, aerial stems are 

 much branched, bearing pinnately compound, trifoliate leaves in 

 an alternate phyllotaxy. The leaflets are linear, oblong or obo- 

 vate-oblong, and toothed toward their apices, with slender, awl- 

 shaped stipules that are adnate to the petiole. (Fig. 157, A.^ 



The Flower and Fruit. — The flowers are borne in short, 

 compact, axillary racemes; and the papilionaceous blossoms are 

 purple or rarely white. (Fig. 156.) In hybrid forms, yellow 

 flowers are occasionally produced; and the sand lucerne, M. media, 

 has flowers which vary from yellow to purple. The floral structure 

 and ontogeny are similar to that of the pea in all essential details. 

 (See Chapter XII.) 



The calyx tube consists of five undiverged sepals terminated by 

 five lobes or teeth that exceed the length of the tube, and the five 

 petals form a corolla of the papilionaceous type in which the 

 standard is somewhat longer than the two lateral wings, these, in 



