24 THE BIOLOGY OF FLOWERING PLANTS 



from the pericycle. They burst through the cortical tissue 

 and appear in a definite number of rows corresponding to 

 the groups of protoxylem, the wood vessels first formed, 

 opposite which they grow out. In the pea, for example, 

 there are four rows, in the beet two. This position favours 

 the rapid transference of water to the conducting tissues of 

 the main root. In the further development of the root 

 system of many plants, such as the lupins, the mallows, the 

 dandelion, the primary root remains predominant and is 

 the centre of a system of branches of the second and higher 

 orders. This, however, is not always the case. 



The most notable departure from this type is shown 

 by the monocotyledons. The radicle does not retain its 

 supremacy beyond the first stages of germination. In the 

 maize it may be identified for several days ; in most cases 

 it is scarcely recognisable. Its place is taken by a number 

 of roots of equal value produced adventitiously, first from the 

 cotyledonary node of the hypocotyl, the seedling stem 

 between radicle and cotyledons, and later from the higher 

 nodes of the stem. The distinction between these two types 

 of root system may be seen very clearly by comparing the 

 common dandelion and the annual meadow grass. 



The later development of the root system is very diverse. 

 In many dicotyledons the main root ceases to be of primary 

 importance very early, side roots of the second and higher 

 orders equalling or exceeding its gro\vth ; an example is 

 the common groundsel. In this, and in many other, dico- 

 tyledons adventitious roots are formed in addition. In 

 Rhizophora and other mangroves the main root fails to 

 develop, and its place is at once taken by rapidly developing 

 side roots. Where vegetative multiplication by runners, etc, , 

 occurs, the root systems of the new plants are entirely 

 adventitious. In many root systems there is a diff"erentiation 

 into long anchoring and conducting roots, and short and 

 short-lived absorbing roots. The two types may also show 

 structural differences (Tschirch, 1905.) Transitions occur 

 between the two. Goebel regards the differences as merely 

 quantitative. 



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