34.] CHAPTER II. THE TISSUES. 189 



Organs developed from the superficial layer alone (dermatogen 

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

 cell; as all sexual organs; sporangia of all Ferns (except Ophio- 

 glossacese and Marattiaceue) i.e. all leptosporaiigiate Ferns and 

 Rhizocarps (Hydropterideae) ; or they may be developed from a 

 group of superficial cells, as the sporangia of the Ophioglossaceaa 

 and Marattiacea3 (eusporangiate Ferns), of Equisetum, the Lyco- 

 podiaceee, Selaginella. 



The antheridia of Fucus are peculiar in that they are single terminal cells of 

 branched hairs. It is a question whether or not the autheridia of Anthoceros 

 (Liverworts) are of endogenous origin. lu Mosses the apical cell of the adult 

 shoot generally give rise to a sexual organ. 



Organs developed from the superficial and deeper layers. In most 

 cases the organ is developed from the superficial and the subjacent 

 (hypodermal) layer, e.g. microsporangia (pollen-sacs) and macro- 

 sporangia (ovules) of most Phanerogams ; in some cases cells of 

 one or more deeper layers take part in the formation of the organ 

 (e.g. sporangia of Isoetes, ovules of Greum, Symphytum, Verbascum, 

 etc.) ; rarely only a single cell of the hypodermal layer is con- 

 cerned (e.g. ovule of Orchids) ; in Balanophora the ovule appears 

 to be developed from a single superficial cell. 



The primitive sporogenous tissue (archesporiurn, see p. 73) is, in the sporangia 

 of all Vascular Plants, derived from the hypodermal layer of the young spor- 

 angium ; it may consist of a single cell, or of a row of cells, or of a layer of cells. 

 In the Mosses the arcl>esporium is more deep'y seated, arising from the ex- 

 ternal layer of the endothecium (rudimentary plerome) as in most Mosses, or 

 from the innermost layer of the amphithecium (rudimentary perihlem) as in 

 Sphagnum and in the Liverwort Anthoceros. 



G. The Development of Adventitious Second art/ Members (see p. 

 21). 



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 formation of a growing-point, by the divi- 

 sion of a group of pericycle-cells in the way described above 

 (p. 186) 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. When they are developed on old stems, they 

 arise from more deeply placed tissues, such as those of the primary 



