ONTOGENY 541 



divisions to some of the secretory cells. The majority <>i' the latter, 

 however, are derived from the other primary mother-cell. Onto- 

 genetically, these glands may be compared to the peripheral fibrous 

 strands in the haulms of Papyrus antiquornm, which also arise partly 

 from protoderm and partly from fundamental meristem. 



Excretory organs rarely originate in the protoderm. The Eu- 

 Crotoneae are furnished with epidermal oil-cells, which sometimes 

 protrude in tubular fashion into the photosynthetic tissue (e.g. in 

 Groton eremopMlus and Crotonopsis alutaris, according to Froembling). 

 Tannin-sacs and other elongated excretory elements occur in the epi- 

 dermis of certain Crassulaceae, Saxifragaceae, Geraniaceae, etc. 

 The cystolith-cells of the Urticaceae and Moraceae, and of some 

 Acanthaceae, are also protodermal in character. 



The origin of secretory and excretory organs from fundamental 

 meristem hardly requires detailed consideration. As a matter of fact, 

 the vast majority of internal glands, such as the various secretory 

 passages, mucilage-, resin- and oil-cells, crystal- and tannin-sacs are 

 derived from ground-meristem. The foliar crystal-cells of Citrus, 

 though apparently belonging to the epidermis, are actually sub-epi- 

 dermal in origin ; as Guttenberg has shown, they attain their final 

 position by active sliding growth, in the course of which they split 

 apart the radial walls of the overlying epidermal cells. In this manner 

 they may push their way as far as the cuticle, and are thus readily 

 mistaken for protodermal elements. Eothert has described a similar 

 state of things in the case of Eichliornia speciosa. According to Knott, 

 another very remarkable instance of this invasion of the epidermis by 

 sub-epidermal elements is furnished by the stinging-cells of Dalc- 

 champia and Tragia which have been described above. Bud. Miiller 

 states that the apparently epidermal oil-cells of Arisfolochia are likewise 

 sub-epidermal elements, which become superficial as a result of sliding 

 growth. 



It is the procambium, finally, which gives rise to the secretory 

 cells of the resin- (or oil-) passages that occur in the primary 

 leptome of the Araucarieae, Clusiaceae and Axacardiaceae, and in 

 the primary hadrome (of the stem) of various Conifers (Pinus, Larix, 

 etc.). According to Ambronn, the cortical oil-passages of Umbelliferae 

 probably always originate together with collenchyma- or mestome- 

 bundles from common procambial strands. In conclusion, it may be 

 remarked that stegmata (cf. above, p. 538) are formed by the septation 

 of peripheral components of procambial strands which I'm- the resl 

 become converted into bast fibres. 



