540 



SECRETORY AND EXCRETORY SYSTEMS 



V. DEVELOPMENT OF SECRETORY AND EXCRETORY 



ORGANS. 



The organs which have been described in the preceding sections of 

 this chapter may arise from any one of the three primary meristematic 

 tissues. 



The mode of origin of secretory organs from protoderm requires no 

 special explanation, at any rate so far as dermal glands are concerned. 



But cases are also known in which glands 

 are internal in a topographical sense, and 

 yet are purely protodermal in origin. 

 According to Von Hohnel, the suit- 

 epidermal glands of Amorpha, Myrtus, 

 and Eugenia belong to this category. 

 In the case of Amorpha, a group of 

 protodermal cells undergo radial exten- 

 sion ; then follow tangential divisions, 

 resulting in the differentiation of secre- 

 tory cells and secondary epidermal ele- 

 ments, separated from one another by 

 one or two intermediate layers. The 

 young gland bulges outwards, its pro- 

 todermal origin being perfectly obvious 

 at this stage. When fully developed, 

 however, " the whole structure is pushed 

 downwards into the mesophyll owing to 

 the tension in the epidermis," and there 

 is nothing to indicate its peculiar mode 

 of development. From the ontogenetic 

 point of view, such secretory organs 

 would be classed among dermal glands 

 (as Von Hohnel has indeed suggested), 

 whereas they are undoubtedly in- 

 ternal so far as actual position is 

 concerned. 



According to Ranter, the sub-epidermal foliar glands of Dictamnus 

 Fraxinella (Tig. 219) occupy an intermediate position between secretory 

 organs which are protodermal, and those which originate in the funda- 

 mental meristem. The glands in question arise from paired primary 

 mother-cells, of which one is protodermal, while the other belongs to the 

 ground-meristem. The protodermal element, first of all, undergoes a 

 tangential division. The outer of the resultant daughter-cells produces 

 secondary epidermal elements, while the inner gives rise by further 



Pig. 219. 



Development of a sub-epidermal gland 

 tie in the upper Bide of the leaf of Uic- 

 tit >n nn.< Praxi m I In. .1. Initiation of the 

 gland; p, c, cells which give rise to the 

 secretory tissue; d, cell which produces 

 secondary epidermal elements. B. Alater 

 stage. C. Fully-developed gland ; o, drop 

 of ethereal oil ; d, secondary epidermal 

 cells. 



