204 



MECHANICAL SYSTEM 



without any preliminary tangential division taking place ; thus, for 

 instance, in the scape of Allium ursinum. In Peperomia latifolia, on 

 the other hand, the outer portion of the subepidermal cylinder of 

 collenchyma owes its origin to the repeated tangential division of the 

 protoderm. 



Fig. 7D. 



Protodermal fibrous strands in T.S. A. Adaxial side of a foliage-leaf of Cyperus 

 l/laber; b, fibrous strand ; e, "secondary" epidermal element. B. Abaxial side of a 

 small bract ; a protodermal bast-fibre is shown, overlain by a "secondary" element 

 of the epidermis. C-G. Papyrus anliqv.orum. C. Fibrous strand from the abaxial 

 side of a bract. D. Fibrous strand from the adaxial side ; the mechanical tissue has 

 evidently arisen from a mother-cell cut out in the middle of a narrow protodermal 

 clement. E, Fibrous strand from the adaxial side (a slightly different case). The 

 greater portion of the protodermal cell has been converted into mechanical tissue ; 

 the smaller portion has only produced three flattened "secondary" epidermal 

 elements. F. Peripheral fibrous strand from the haulm ; its outer portion is pro- 

 todermal in origin, while its inner portion is derived from fundamental meristem. 

 6. A similar strand in the procambial stage. 



The sclerenchymatous layers of seed-coats and pericarps are also in 

 many cases {e.g. among the Leguminosae) protodermal in origin. 



3. Development of stercome from fundamental meristem. 



In some Monocotyledons, and particularly among the Liliaceae, 

 the fibrous cylinder originates in the fundamental meristem, certain 

 cells of which undergo repeated division and so give rise to a secondary 

 meristem ; the latter cannot be regarded as a typical procambium, 

 because its cells are often not prosenchymatout in character, and also 

 because it contains very well developed intercellular spaces. 



The fundamental meristem is still more frequently responsible for 

 the production of collenchyma ; in this case also the intervention of a 

 secondary meristem is the rule. 



