136 PART II. ANATOMY AND HISTOLOGY. [ 34 



the relation of these members to the primary meristem, only arises 

 with reference to those plants, the bodies of which consist of many 

 layers of tissue ; their origin in plants, the bodies of which consist 

 either of filaments, or of flattened expansions of a single layer of 

 cells, need not be considered here. 



In the cases under consideration, the reproductive organs may 

 be developed either from the superficial layer alone, or from that 

 and one or more of the subjacent laj'ers. 



Organs developed from the superficial layer alone Jderinatogen 

 when differentiated) : these may be developed each from a single 

 cell, as all sexual organs, and the sporangia of all leptosporangiate 

 Ferns and Rhizocarps (Hydropteridese) ; or they may be developed 

 from a group of superficial cells, as the sporangia of the Ophio- 

 glossacese and Marattiacese (eusporangiate Ferns), of Equisetum, 

 and of Lycopodinse. 



Organs developed from the superficial and deeper layers. In 

 most cases the organ is developed from the superficial and one 

 or more of the subjacent layers, e.g. microsporangia (pollen-sacs) 

 and macrosporangia (ovules) of most Phanerogams. 



The primitive sporogenous tissue (archesporium, see p. 53) is, in the 

 sporangia of all Vascular Plants, derived from the hypoclermal layer of 

 the young sporangium ; it may consist of a single cell, or of_a row of 

 cells, or of a layer of cells. In the Mosses the archesporium is more 

 deeply seated, arising from the external layer of the endothecium 

 (rudimentary plerome) as in most Mosses, or from the innermost layer of 

 the amphithecium (rudimentary periblem) as in Sphagnum and in the 

 Liverwort Anthoceros. 



G. The Development of Adventitious Secondary Members (see 

 p. 9). 



1. On the stem. The most common case is that of the develop- 

 ment of roots, but occasionally shoots (buds) are developed ad- 

 ventitiously. 



The adventitious development of roots on the stem takes place 

 most commonly by the division of a group of pericycle-cells to 

 form a growing-point, in the way described on page 134 with 

 reference to the development of normal lateral roots on the 

 parent root. In any one plant the two processes are similar in 

 every detail. 



The adventitious development of biids on the stem may take 

 place either exogenously or endogenously. In the former case__the 

 buds may be developed each from a single epidermal cell (e.g. 



