374 



PRIMARY ARRANGEMENT OF TISSUES. 



reticulations and pits, or with spiral fibres ; one or two layers of delicate cells ensheath 

 the whole end of the bundle, and separate it from the thin-walled epidermis of the 



furrow. 



The ends of the bundles of the leaf-teeth of Drosera, and of the inner surface of 

 the foliar pitchers of Nepenthes, are likewise placed close under specialised portions 

 of the epidermis, and in point of structure stand next to those just described. 



The leaf of species of Drosera (in particular of D. rotundifolia) has at its edge and 

 on its entire upper surface numerous filiforp teeth with broadened ends'. Those of 

 the surface are, apart from diflFerences of length, similar to one 

 another; they are filiform processes, somewhat conically 

 tapered, but swollen at the end to form an approximately 

 ovate head. They consist of some layers of elongated cells, 

 in the middle of which are one or rarely two narrow spiral 

 vessels or rows of tracheides (I will not state definitely which) 

 branching off from the net of bundles of the leaf-lamina, and 

 having a straight course ; the whole is covered by a single epi- 

 dermal layer, the cells of which are likewise elongated. In the 

 middle of the knob-shaped end the spiral vessel enters a group 

 of closely-connected, short, reticulated and spiral tracheides, 

 which has, as a whole, an ovate form, and constitutes the main 

 bulk of the terminal portion. The layer of cells surrounding the 

 veszeis terminates below the middle of the group of tracheides, 

 in the form represented in Fig. 176. At the point of transition 

 to the head the epidermis first becomes short-celled, and then 

 suddenly passes over into the three-layered covering of the sur- 

 face of the knob, which, as Warming has shown, is derived partly 

 from the primary epidermis, partly from the layer of meristera 

 lying below the latter. The innermost layer of this covering 

 forms a bell-shaped single stratum, consisting mostly of elon- 

 gated cells, which is in immediate contact above and at the 

 sides with the group of tracheides, while below, at the edge 

 of the bell, it ends in the outer surface of the epidermis. 

 The membranes of its cells are smooth and firm, similar in 

 their reactions to those of an endodermis, the walls vertical 

 to the surface being undulated. From the edge of the bell 

 onwards the surface of the knob is covered by two layers of 

 cells, which are thin-walled, and are distinguished in the fresh 

 condition by their dense, intensely red contents. The inner 

 of these two layers does not reach quite to the edge of 

 the bell, and consists of small', isodiametric, polyhedral 

 cells, which are very delicate, and are in uninterrupted connection one with another. 

 It is everywhere covered by the outermost layer, which is continued immediately 

 from the edge of the bell over the whole surface of the knob, and consists of 

 polygonally prismatic cells in uninterrupted connection. The diameter of these cells ver- 

 tical to the surface increases successively towards the apex of the knob ; here it is about 

 twice as long as the diameters coinciding with the surface, while at the base it is about 

 equal to them. The delicate outer walls of these layers, which are covered later on with 

 the sticky secretion (p. loi), show a very delicately undulated outline at the edges. — The 

 teeth of the edge of the leaf are expanded at their ends to the form of a spatula or long 



FIG. 176 (145). — Drosera rotundirolia. 

 End uf a tooth from the upper side of 

 the leaf; axial longitudhial section. 

 s — .f tile bell-shaped layer directly sur- 

 rounding the end of the bundle ; its 

 lowest, very lon^f cells, s, standing with 

 their narrow oyter wall in the epidermal 

 surface, belong, as shown by Warming, 

 and as their appearance indicates, to 

 tlie printqry epidermis : all the others, 

 occurring above theapex, proceed fron; 

 the subepidermal meristem. Tlie same 

 holds goQ(^ of the cells pf the second 

 layer occurring above khe apex : in so 

 fai? as they border on the apical cells 

 of the innermost layer thej proceed 

 from the gister-cells of thtj. latter. 

 Those, on the other hand, which belong 

 to the lowfer edge of the knob, as wellas 

 the entire oi^^ermost layer, are derived 

 from theprin\ary epideniiis. 



' Compare Meyen, Secretionsorgane, p. 51. — Trecul, Ann. Sci. Nat. 4 ser. torn. III. — Nitschke, 

 Botan. Zeitg, i86t, Nos. 33, 33, &c.— Martinet, Ann. Sci. Nat. 5 ser. torn. XIV. — Warming, I.e.; 

 compare above, p. 57. — Darwin, Insectivorous I'lants. 



