1883.] development of the Leaves of Pinus Silvestris L. 349 



immediately underlie. Except in the regions where these cells 

 underlie the mother-cells of the stomata the layer is usually two 

 cells thick and may even attain a depth of three cells at the ends. 

 They long retain their protoplasmic contents and nuclei but never 

 contain chlorophyll grains. By some e.g. Hildebrand, Haberlandt 

 &c. they have been called "liber-cells" or "sub-epidermal bast cells," 

 since in general characters they somewhat resemble bast-fibres but, 

 as MacNab 1 has alread}^ pointed out, they have no relation what- 

 ever to the fibrovascular bundle, of the phloem of which the true 

 liber-fibres when they occur always form an integral part, being 

 simply sub-epidermal in position. Similar cells also occur in 

 Lycopodium. The hypoderma in the mature leaf is conspicuous 

 and forms a continuous band except beneath the stomata. 



B. Cortical. The cells are arranged in transverse section in 

 several (2 — 3) concentric rows extending round the whole periphery 

 of the leaf and in most cases contain two or more nuclei : in the 

 earlier stages they are somewhat polygonal in transverse section, 

 flattened and columnar in shape, and of large size. Their nuclei 

 lie centrally or nearly so, and are connected with a protoplasmic 

 sac (primordial utricle), which closely lines the walls on all sides 2 , 

 by means of very numerous fine and delicate filaments or strands 

 of protoplasm passing across the cell vacuole so that the nuclei 

 present a stellate form 3 . 



The cells are united together parenchymatously, but are closely 

 packed in such a manner as to form a most perfect palisade, 

 no inter-cellular spaces being at this time observable. At a some- 

 what later period one or several of these protoplasmic strands may 

 become converted into cellulose, and thus by this means each of 

 the nuclei is firmly attached to the wall. The change is easily 

 demonstrable by the action of staining reagents, and for this 

 purpose I have used aniline colours 4 and also dilute tincture of 

 Iodine followed by Chlor. Zinc. Iod. (Schultze's) solution; in the 

 earlier cases all the strands invariably stain after treatment with 

 Hoffmann's Blue or Aniline Blue, and turn Brown with Chlor. Zinc. 

 Iod., while in the later stages those filaments which have become 

 converted into cellulose refuse to become stained with the two 



1 Histological Notes, iv. "The thickened cells in the leaves of Pinus," Jour. 

 Bot., Vol. x. 1872, p. 33. Also Trans. Bot. Soc. Edin., Vol. xi. 1872, pp. 296, 

 2<J7. 



2 I was enabled to demonstrate the presence of this latter by plasmolysing fresh 

 material and also by staining thin sections of absolute alcohol material with Hoff- 

 mann's Blue. 



3 Gardiner has recently shown that the occurrence of similar filaments connecting 

 its ends to the primordial utricle causes the elongated shape of the nucleus in 

 Spirogyra, an observation which I have been able fully to confirm. 



4 The method I have employed is to dissolve the dye in Absolute Alcohol diluted 

 to 50 per cent, strength ; stain the sections in this, then wash with water and mount 

 in strong pure Glycerine. 



25—2 



