I02 



MORPHOLOGY OF TISSUES. 



vcloping scales of leaf-buds {Aesculus), on stipules which precede the leaves in de- 

 velopment {Cunonia, Viola, Prunus), on ochreae (Polygonaceae), or on young leaves 

 themselves {Ribes, Syringa). The secretion of the coUeters is a watery mucilage in 

 the Polygonaceae ; in the rest it is mixed with drops of balsam or resin. The gum- 

 mucilage always arises from the conversion of a layer of cellulose lying beneath Jhe 

 cuticle of the colleter, the substance of which swells on addition of water, and raises 

 the cuticle in p'aces into small bladders (Rumex), or detaches it continuously from 

 the hair as a larger bladder; finally the cuticle bursts, and the mucilage escapes and 

 flows over the bud; the uninjured inner layer of cell-wall can, on its part, form 

 a cuticle, beneath which a layer of cellulose again separates, and the process is repeated. 

 Where balsam is also excreted, it may be recognised even in the cells of the hair; 

 but it appears outside the cell-wall in dro s as a deposit in the mucilage, or forms 

 the basis of the secretion. Frequently also the young epidermis itself between the 

 colletcrs participates in these processes (Polygonaceae, Cunonia) ; and the blastocolla is 

 even produced exclusively from the epidermis; thus arises, for instance, the greenish 

 balsam on the bud-scales and foliage-leaves of poplars'. 



FIG. 84. — Development of the stomata oi Pteris flabellata (seen from the surface) ; A very young epidermal cells; 

 B nearly mature ; v cell formed by the preliminary division ; A s mother-cell of the two gfuard-cells s svs\ B. 



The Stomata^ are never found on the epidermis of true roots; on the other 

 hand they are usually present on underground stems and leaves ; according to Borodin 

 they are occasionally found even on submerged parts; but they are formed in the 

 largest numbers on the aerial internodes and foliage-leaves, though not altogether 

 absent from the petals and carpels; they even occur in the interior of the cavity of 



^ [Some reference should here be made to the remarkable discovery by F. Darwin (Quart. 

 Journ. Micr. Sci. 1877, p. 245) of the protrusion of protoplasmic filaments from the glands within 

 the cup formed by the connate bases of the leaves of Dipsacus sylves(ris, which he believes to have a 

 function connected with the absorption of nitrogenous matter for the nutrition of the plant] 



2 H, von Mohl, Verm. Schriften hot. Inhalts. Tubingen 1845, pp. 245, 252, — Ditto, Bot. Zeitg. 

 185^), p. 701. — A. Weiss, Jahrb. fur wass. Bot. vol. IV, 1865, p. 125. — Czech, Bot. Zeitg. 1865, 

 p, loi. — Strasburger, Jahrb. fiir wiss. Bot. vol. V. 1866, p. 297. — E. Pfitzer, ibid., vol. VII, 1870, 

 p. 5;^2. — Rauter, Mittheil. der naturwiss. Vereins fiir Steiermark, vol. II. Heft 2, 1870. — 

 Borodin, Bot. Zeitg. 1870, p. 841. — Hildebrand, ibid., p. i. — Ditto, Einige Beobachtungen aus 

 dem Gebiete der Pflanzenanatomie. Bonn 1861. — Prantl, Ergebnisse der neuern Untersuchungen 

 iiber Spaltcffnimgen, Flora 1872. 



