RUDIMENTARY AND TRANSFORMED BRANCHES. 153 



the structure of the stem becomes much disguised. In the case 

 of parasites like Phoradendron, which live for several years, a 

 vertical section through the stem of the host-plant shows how 

 complete the union is between the host and parasite. The junc- 

 tion has been well compared to that which takes place between 

 a scion and its stock, since the newer-formed tissues of both 

 plants become perfectly united, and their subsequent growth 

 goes on together. 



428. The relations of the root to the stem are not complicated, 

 except as regards the bundles at the " crown" of the root, or the 

 point where it meets the stem. When the primary structure of 

 dicotyledons in which the liber of the root is arranged in one 

 waj r and that of the stem in another, as shown in Figs. 92 and 

 112, pages 111 and 137, is followed by the formation of a true 

 cambium ring, the subsequent growth of root and stem is alike. 

 Yearl}* additions are made in the root in the same way as in the 

 stem; but owing to the unequal resistance exerted by the soil, 

 such increments are often very irregular. 



Roots may be produced at an} r part of a stem where adequate 

 moisture and warmth are furnished ; but they strike off chiefly 

 at nodes, and, in the case of cuttings, also at the seat of injury 

 where the callus is formed. Such secondary roots form on stems 

 in much the same manner as root- branches do upon roots. 



429. Rudimentary and transformed branches present few ana- 

 tomical difficulties. In the structure of a branch tendril, or 

 runner, it is generally easy to recognize the degree of reduction 

 which the normal fibre-vascular system has undergone. In the 

 case of underground stems and branches there are often puzzling 

 anomalies, but they can mostly be explained by the following 

 facts brought out by Costantin, 1 who has made a special study 

 of a large number of rhizomes: 1. The epidermis, if present, 

 is modified by becoming cutinized first on its outer walls, where 

 it may acquire considerable thickness, and later on its lateral and 

 internal walls. 2. The cortex increases either by enlargement 

 of its cells or by their multiplication, the collenchyma diminish- 

 ing or completely disappearing. 3. A cork -layer is sometimes 

 produced at an early period, from different points in the epi- 

 dermis, in the cortical parenchyma, in the endodermis, in the 

 peripheral layer of the bundles, or, lastly, in the liber. This 

 replaces to a great extent the fibrous layer which is so com- 

 mon in aerial, but never much developed in underground stems. 



1 Ann. des Sc. nat., ser 6, tome xvi., 1883, p. 164. 



