i8o 



strong, hard, wood body with a thin, tender bark. The cells of the wood 

 tissue, like all the other wood cells, are thick-walled, spindle-shaped and 

 wedged between one another. In the cultivated root, instead of these wood 

 cells, thin-walled, short cells are present, ending almost bluntly against one 

 another and even the ducts which lie in scattered groups between the par- 

 enchymatous cells are but little lignified. The latex tubes already formed in 

 the bark, when spiral porous ducts are produced in the wood body, have 

 broadened like all the cells of the bark. Instead of the starch which, in the 

 wild carrot, fills out the whole bark tissue, occurring here and there in the 

 wood body also and increasing to 70 per cent, of the dry weight, sugar has 

 been formed usually in good table carrots so that only traces of starch may 

 be found. The better the variety, the less the starch content as in the 

 Dutch pale yellow and the Duwicker carrot. Gradual transitions are found 

 back toward the wild plant in other cultural varieties used as fodder, such 

 as the Altringham carrot and the white horse carrot. Specimens of all 

 varieties found on poor soil go to seed as a rule in the autumn and are dis- 

 tinguished by a thin, often divided, root which, because of its lignification, 

 recalls clearly the ancestral wild carrot. The same behavior is character- 

 istic of the turnip-rooted cabbage, Swedish turnip, radishes, Kohlrabi, etc. 



These differences are best made clear by comparing the anatomical 

 structures. In Fig. 21 is shown a longitudinal section through a two-year 

 old wild carrot. In this figure a is the vertically elongated parenchyma of the 

 pith-like central part with scattered spiral, porous ducts ; b the xylem, made 

 up of spindle-like wood cells together with ducts and the part of the medul- 

 lary ray which extends toward the secondary cortex ; c the cambium which 

 has become an elongated, thin-walled parenchyma ; d the secondary cortex 

 with its resorption spots which follow the course of the latex ducts; e the 

 primary cortex ; / cork. 



Fig. 22 is a corresponding section from a two-year old cultivated carrot. 

 The letters in both figures indicate the same parts and a comparison of the 

 similarly designated tissues makes very clear the change in the wood tissue 

 and the increase in the dimensions of the secondary cortex in the cultivated 

 carrot. 



In all tuberous vegetables lignification also occurs normally when they 

 grow too old and then this process, as in individuals lignifying prematurely, 

 is accompanied by a partial disappearance of the sugar. 



It is well-known, from experience, that many of our vegetable plants 

 lignify in hot climates. Precautions against this latter condition will be hard 

 to find since the tropical warmth and excess of light favor rapid lignification. 

 In cultivation in temperate climates, lignification can certainly be avoided by 

 abundant watering and fertilizing; — only care should be taken in this that 

 the land is deep and the seed good. Special attention should be given to 

 the choice of seed, because seeds from dry localities carry with them a great- 

 er tendency to lignification and to a repeated division of the root. 



