1906] On the Structure of Roots. 21 



leptome. The cambium commences then to develop new groups 

 of leptome outwards and new groups of hadrome inwardly. By 

 continued growth the cambial arches extend towards the pericycle 

 and meet outside the rays of the hadrome, thus a completely closed 

 ring of cambium becomes formed, and this is able to develop lep- 

 tome and hadrome throughout its entire width. The original 

 structure of the root has, thus, become very considerably changed, 

 since the secondary groups of leptome are located in the same 

 radius as the secondary hadrome, while the primary were arranged 

 in alternation with each other. At this stage the structure is very 

 much like that of a stem (Dicotyledones and Gymnosperms) ex- 

 cept that the primordial hadromatic rays are yet to be observed. 

 But besides resulting in the formation of secondary leptome and 

 hadrome due to the cell-division of the cambial ring, the pericycle 

 possesses, also, the power of developing secondary tissues by 

 similar cell-divisions. This new tissue is, on the other hand, 

 parenchymatic, and is called the secondary cortex, since it agrees 

 in all respects with this particular tissue. It is easy to under- 

 stand that the primary cortex with epidermis and endodermis are 

 not able to follow the continued growth of the elements in the 

 central cylinder, but become split, die off and are finally thrown 

 off altogether, thus the secondary cortex formed by the pericycle 

 takes the place of the primary. 



We may pass now to the structure of the beet. In a fully 

 developed root of this plant we notice in a cross-section a number 

 of concentric rings, resembling the annual rings of a perennial, 

 woody stem. However, these rings are all made in one summer, 

 and by following their structure gradually from month to month, 

 the structure is shown to have originated in a very different way 

 from that of a stem. The fact is that the secondary cortex is here 

 able to develop continuously new strata of leptome and hadrome 

 separated by medullary rays in concentric rings and in centrifugal 

 direction. As soon as one stratum of leptome and hadrome has 

 performed its function for some time, it ceases to grow any fur- 

 ther, and a renewed formation of another ring outside the first 

 one takes place and so on, so that a number of rings are formed 

 during the season ; the most conspicuous portion of each of these 



