374 



PRIiMARV 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. rotundil'olia) has at its edge and 

 on its entire upper surface numerous filiform teeth with broadened ends\ Those of 



the surface are, apart from differences 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 

 ves els terminates below the middle of the group of tracheides, 

 in the form represented in ¥\g. 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 meristem 

 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-wMlled, 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 coinc-iding 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 rotundifolia. 

 End of a tooth from the upper side of 

 the leaf; axial lon.i^itudinal section 

 J — s the bell-shaped layer directly sur- 

 rounding the end of the bundie ; its 

 lowest, very long cells, s, standing with 

 their narrow outer wall in the epidermal 

 surface, belong, as sho«n by Warming, 

 and as their appe.arance indicates, to 

 the primary epidermis; all the others, 

 occurring above the apex, proceed fron; 

 the subepidermal meristem. The same 

 holds good of the cells of the second 

 layer occurring above the apex : in so 

 far as they border on the apical cells 

 of the innermost layer they proceed 

 from the sister-cells of the latter. 

 Those, on the other hand, which belong 

 to the lower edge of the knob, as well as 

 the entire outermost layer, are derived 

 from the primary epidermis. 



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

 ])otan. Zeitg. 1861, Nos. 22, 23, &c.— Martinet, Ann. Sci. Nut. 5 ser. loni. XIV. — Warming, I.e.; 

 comj'jare above, p. 57. — Darwin, Insectivorous I'lanls. 



