8i2 A TEXTBOOK OF THEORETICAL BOTANY 



in Asparagus. Frequently, as in Oenanthe crocata, these tubers are the means 

 whereby the plant outlasts the winter season (Fig. 808). 



Plants which belong to the so-called " root crops " are in a special 

 categor}'. Nearly all of these, such as the Carrot, Turnip, Beetroot, etc., are 

 biennials (Fig. 809). In the first season the storage organ is formed, with 



Fig. 807. — Dahlia variabilis. Cluster of storage 

 root tubers producing fibrous roots. 



only a very contracted stem which remains at soil level, bearing a group of 

 large leaves. In the second season the stem elongates and bears flow'ers, 

 drawing on the stored food below ground. The plant thus loses much of its 

 food value to man, if it is allowed to flower or " bolt," as gardeners say. 



These storage organs are compound structures, the lower part being the 

 true primary root, and the upper part being the hypocotyl of the seedling. 

 The two parts are completely united into one organ. The nature of the 

 storage tissue varies. In the Turnip and the Radish it is formed from 

 parenchymatous secondary xylem. In Carrot and Parsnip it consists of 

 parenchymatous tissue which chiefly represents secondary phloem and 

 secondary cortex. In the Beetroot and Mangold it is largely soft xylem 

 tissue in concentric rings, alternating with parenchymatous phloem. The 

 rings are formed by a succession of short-lived cambia which are produced by 

 the pericycle. The same structure occurs in many storage roots (Fig. 810). 



