6zi THE STRUCTURE OF ECONOMIC PLANTS 



of the four principal types. This is illustrated by Morse (i8), 

 who has compiled a list of approximately iioo names that have 

 been applied to the lettuce varieties in this country. This involves 

 tremendous duplication as there are probably less than 150 distinct 

 varieties even on the basis of minor differences. In order to 

 clarify the situation, Morse has described the forms w^hich he 

 regards as constituting the standard American varieties. Twenty- 

 two types are included, the most important being the New York 

 variety which, with its sub-type Imperial, is estimated to represent 

 approximately half of the commercial lettuce crop grown for 

 shipment. 



GENERAL MORPHOLOGY 



The lettuce plant is an annual; and, as the generic name implies, 

 one of its important characteristics is the presence of a milky juice 

 or latex. In head lettuce, the plant may produce heads in 65 to 

 90 days depending upon the strain used; while in California, where 

 a majority of the seed is grown, the crop is planted in late December 

 or January and the fruits begin to ripen early in August. (Fig. 



32-3-) 



The Root. — The plant has a strong fleshy tap root which 



develops very rapidly. Weaver and Bruner (19) have described 

 the development of the root system and find that under favorable 

 conditions the tap root may elongate at the rate of an inch per 

 day, and, in some soils, may attain a maximum depth of over 6 

 feet by the time that the flower stalks are producing blossoms. 

 The average depth is approximately 5 feet at maturity, and the 

 major portion of the entire system occupies the first x feet of soil. 

 The principal lateral roots arise in two rows chiefly in the first 

 10 inches of soil, extending outward for 6 to 18 inches and then 

 downward. There are few laterals originating below the first 

 foot of soil and many of the lower ones are short and thread- 

 like. Those which arise from the upper portion of the tap root 

 may equal it in length, and like it attain a diameter of an inch or 



more. 



The Shoot. — At first, the plant develops a fleshy crown stem 

 from which basal leaves arise in an alternate spiral phyllotaxy, 

 the degree of compactness varying with the variety. (Fig. 3x4.) 

 In some instances, they form such a firm, solid head that in the 

 production of seed it must be partially cut open in the field in 



