98 CELLULAR TISSUE. 



twice as tliick as in these. The small polygonal outer walls belong to narrow sac- 

 like cells, of which the peripheral ones (14 to 24 in number) run to the inner pole, 

 curved like meridian lines. They are uninterruptedly connected laterally, thus forming 

 with one another the wall of a hollow sphere : the 5-8 central ones are sinuous or 

 bowed, and run through the cavity of the hollow sphere : at the inner pole they are 

 applied with their widened ends to one another, and to the peripheral series, and 

 thus close the hollow sphere (Fig. 42, a). Between the sides of the central and the 

 inner surfaces of the peripheral cellulose walls lie wide interspaces filled with large 

 masses of resin, or innumerable resin drops suspended in transparent fluid (' Milchsaft,' 

 Hildebrand, /. c). The contents of the sac-like cells are, when old, of a similar 

 nature : at first they contain clear protoplasm, which is scarcely granular, and watery 

 cell-sap. 



As distinctly indicated by the mature condition, these glands originate from a single 

 primary epidermal cell, which bulges inwards, and is repeatedly divided by walls 

 perpendicular to the surface. The segments elongate in the same direction : their 

 lateral cellulose walls are at first uninterruptedly connected {b, Fig. 42). During the 

 elongation resin appears at the limiting surface, at first as an homogeneous narrow and 

 short intramural layer, at the middle of the cells, forming transverse rings round the 

 central ones (c, Fig. 42) : gradually it increases in height and thickness, till it attains 

 the size of the large interstices above described. At first, and during the most active 

 growth of the resinous layers, the cells contain a very thin transparent protoplasmic 

 lining, and quite clear colourless cell-sap. 



The secretion of the dermal glands of all categories is in most cases resin, or a mixture 

 of resin and ethereal oil: e.g. Betula, Humulus, Labiatae, Primula, &c. A complete 

 enumeration of the chemical relations of these bodies cannot be undertaken here. In 

 other cases it consists of bodies which swell and dissolve readily in water (vegetable 

 mucilage, gum), as for instance in the buds of the Polygonums (Hanstein, /. c.) ; or of 

 mixtures of these and resin, as, according to Hanstein, in most leaf-buds, e.g. Cunonia, 

 Viola. More rarely, on parts which do not belong to the flower, it consists of mixtures of 

 gum and sugar: thus in Viburnum Tinus, Clerodendron fragrans, where the sugar may even 

 crystallise, Prunus Laurocerasus, Acacia, &c. (Von Schlechtendal, Caspary, Unger, /. c). 

 The resinous secretions may be rather solid, e.g. Aspidium ; but certainly in the majority 

 of cases they are soft and sticky : those which swell and dissolve easily in water are 

 normally always very watery ; most secretions are therefore naturally moist and sticky. 

 The character of the surface is not influenced when the moist secretion remains in 

 the glands, and when only its volatile constituents evaporate through the membrane. 

 This is the case in all intramural glands, and those with tough resistent cuticle, as in 

 Humulus and most Labiats, 



In the bladder-like glands with delicate cuticle the latter is either burst by internal causes, 

 e.g. by increasing aggregation of the secretion, as in the sugar glands of Clerodendron, 

 Acacia marginata, the glands of many leaf-buds ; or rupture of the cuticle results readily 

 from external lesion, and the surface thus becomes sticky through the escape of the 

 secretion. On hair-structures of mature parts the secretion soon ceases after rupture of 

 the cuticle, and the glandular cells dry up : fresh secretion may in that case proceed from 

 other later-developed glandular hairs. In the glandular spots, which lie over the ends of 

 the vascular bundles, the secretion appears in many cases, at all events, to continu3 long 

 after the rupture of the cuticle; still we must in these cases distinguish more exactly 

 how much of that which is excreted is due to secretion itself, and how much to water 

 which filters from the vascular bundles. In unfolded foliage-buds, according to Hanstein, 

 the rupture of the cuticle is often followed by its renewal and the formation of new 

 secretory layers. 



Like hair-structures generally, the glandular hairs are also in many cases transitory 

 organs, w^hich are present in the bud, but disappear after the bud unfolds. The 

 glandular epidermis of many leaf-teeth also secretes while in the bud, and afterwards 



