66 CELLULAR TISSUE. 



character, arranged in two irregular rows, and is borne by a bi-seriate shaggy hair, 

 as stalk. As in Piper there are also found small club-shaped shaggy hairs, from the 

 swelling of which the pearls might have been derived. 



The pearls of the above-named Ampelidea: are, on the other hand, emergences. They 

 consist of several large cells of the character above-stated, and are covered by a protrusion 

 of the epidermis consisting of numerous relatively small hyaline cells. On or near the 

 summit of the body is a stoma, which is widely open, and on old specimens is further 

 extended by rupture at the corners of the slit. Young specimens are seated on the 

 surface as blunt warts, with broad base. In old specimens the upper part swells so much 

 that the point of insertion appears as a relatively small stalk. The pearls of Urtica 

 macrophylla, and, according to INIeycn's statement, of the other Urticacea:, have in the 

 main a like structure, with the ditference that they are without the stoma. The pearls 

 of Cecropia and Bauhinia are, according to Meyen, similarly composed ; they are also 

 M-ithout the stoma, and differ further in their tissue consisting throughout of small very 

 numerous cells. 



V. Prickles and Warts. It was above stated that the massive outgrowths termed 

 prickles and warts are mostly emergences, in the formation of which epidermis and 

 subepidermal tissue conjointly take part. For the majority of these structures, as the 

 prickles of species of Dipsacus, Rosa, Gunnera, Smilax, Solanum, and Ribes, the 

 Cactaceae, &c., this is thoroughly proved by late investigations of the history of their 

 development (Rauter, Kauffmann, Warming, Delbrouck, Uhlworm). Delbrouck and 

 Uhlworm, however, have both shown that exceptions to the predominant law exist, 

 since the prickles of the investigated species of Rubus (R. ca'sius, idaeus, Hofmeisteri) 

 and those of the petiole of Chamxrops humilis belong to the epidermis, and differ only in 

 form and consistence from shaggy hairs. Further that the warts on the foliage leaf and 

 carpels of Bunias Erucago are at least chiefly derived from the epidermis. If once an 

 anatomical distinction is adopted, it is necessary to separate the above outgrowths of 

 epidermis from emergences of the same or similar form, however closely they may 

 correspond to them— and to many sorts of hair-formations of most simple structure — 

 as regards their physiological or teleological significance. 



The often-described oval warts of Dictamnus^, which bear on their apex a short, 

 septate hair, are connected with the above forms. They will be described more in 

 detail below (p. 69). 



2. STRUCTURE OF THE ELEMENTS OF THE EPIDERMIS. 



(a) Protoplasm and Cell-Contents. 



Sect. ii. The wall of the Epidermal cells both in one-layered and maily- 

 layered epidermis encloses as a rule a delicate protoplasmic sac with distinct nucleus, 

 and within this clear transparent cell-sap, which is either colourless, or tinted with 

 dissolved pigments (Erythrophyll, &c.). It is to this condition (and the colourless 

 membrane) that most epidermal layers owe their great transparency. 



In the majority of cases chlorophyll and starch are absent from epidermal 

 cells "-. This is the case without exception in land plants where the tissue is very 

 thick-walled, and surrounded by air ; often also in thin-walled cells occurring under 

 similar conditions. But in not a few other land plants more or less numerous 

 chlorophyll grains, eventually with included starch, lie in the peripheral protoplasm. 



* Meyen's (Secretionsorg.) ' Mlitzenformige Driisen.' Compare Ilofmeister, Pflanzcnzelle, p. 

 259 ; Rauter, /. c. Taf. V, VI. -^ [Cf. Stcihr, Bot. Ztg. 1879, p. 581.] 



