102 
MORPHOLOGY OF TISSUES. 
veloping scales of leaf-buds {Aesculus), on stipules which precede the leaves in de- 
velopment {Cunonia, Viola, Pru?ius), on ochreae (Polygonaceae), or on young leaves 
themselves {Ribes, Syringa). The secretion of the colleters 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 the 
cuticle of the coUeter, the substance of which swells on addition of water, and raises 
the cuticle in places 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 
colleters 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 o{ Pterisflabellata (seen from the burface) ; A very young epidermal cells; 
B nearly mature ; v cell formed by the preliminary division ; A s mother-cell of the two guard-cells s s iw 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. Sei, 1877, p. 245) of the protrusion of protoplasmic filaments from the glands within 
the cup formed by the connate bases of the leaves of Dipsamis sylvestris, 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 bot. Inhalts. Tübingen 1845, pp. 245, 252. — Ditto, Bot. Zeitg. 
1856, p. 701.— A, Weiss, Jahrb. für wiss. Bot. vol. IV, 1865, p. 125, — Czech, Bot. Zeitg. 1865, 
p. loi. — Slrasburger, Jahrb. für wiss. Bot. vol. V. 1866, p. 297. — E. Pfitzer, ibid., vol. VII, 1870, 
p. 5.^2. — Rauter, Mittheil, der naturwiss. Vereins für Steiermark, vol. IL Heft 2, 1870. — 
Borodin, Bot. Zeitg. 1870, p. 841. — Hildebiand, ibid., p. i. — Ditto, Einige Beobachtungen aus 
dem Gebiete der Pflanzenanatomie. Bonn 1861, — Prantl, Ergebnisse der neuern Untersuchungen 
über Spaltöffnungen, Flora 1872. 
