356 



Transactions. 



There is a clear indication that the leaf-trace bundles are sent inwards. 



The Root. — The stele is diarch ; the bundles of phloem are quite distinct 

 and easily distinguished from the xylem. As in the majority of roots, the 

 medulla becomes obliterated. 



In old roots the structure resembles that of the stems, in that secondary 

 xylem and phloem are developed from an extra-fascicular cambium. It 

 differs in that phloem islands in the root are slightly larger than those in 

 the stem, and the fibrous cells round them have thinner walls. The dis- 

 tinction between one season's growth and the next is more apparent. 



A cross-section of the hypocotyl shows two groups of xylem which 

 converge to form, in the root, the plate, on each side of which is the phloem 

 group (see fig. 5, a). 



The development of the root takes place as usual. 



Seedlings have numerous long delicate hairs, the outline of which is 

 often crinkled and wavy. They present a curious resemblance to fungal 



b 



c 



Xylem 



Phloem 



Phloem, 

 islands 



Tracheae. 

 Xylem 



Rootlet 



Fig. 5. — a, Transverse section of root of an old plant; b, transverse section of young 

 branch, near tip ; c, transverse section of part of internal structure of root 

 of a seedling. 



hyphae, and portions of them are often swollen, especially the tip. Some 

 of them are as much as 2 mm. in length, while the root is only 0-25 mm. 

 in diameter. They extend along the root, from just behind the growing- 

 point to the base of the hypocotyl. 



Cotyledons. — Stomata occur on the upper and lower surfaces, and are 

 placed as in leaf and leaf-base, but are not sunk beneath the epidermis. 

 The guard-cells are short and wide, so that in surface view the stomata 

 appear circular. 



Epidermis. — Surface view of epidermal cells shows that they are wavy 

 in outline. In a cross-section the radial walls are shorter than the tan- 

 gential, and the outer walls are flat. Seedlings grown in a greenhouse 

 showed chloroplasts, few in number, in some of the epidermal cells. 



In cotyledons palisade tissue is developed beneath the upper epidermis 

 only. 



Secondary Growth . 



Stems of Salicornia increase greatly in thickness owing to secondary 

 growth, some of the older ones being | in. in diameter. 



The cambium in the original collateral nbro-vascular bundles soon 

 becomes exhausted, causing the secondary phloem and xylem to have an 

 unusual origin. As a rule, with the exceptions noted below, interfascicular 

 cambium is not formed. 



