PROCAMBIAL STEREOME 190 



The hollow stipes of Agarics and certain other Fungi must be regarded 

 as inflexible structures, although they are devoid of special mechanical 

 hyphae. The tubular sporophores (podetia) of Clad on in are provided 

 with a sharply differentiated mechanical cylinder composed of special 

 thick-walled hyphae (Fig. 7 1 b). 



VII. ONTOGENY OF THE MECHANICAL SYSTEM. 



The skeletal system of a plant may originate from any or all 

 of the three primary meristematic layers of the growing-point ; this 

 statement applies equally to bast and to collenchyma, so that the 

 two principal types of mechanical tissue are closely connected onto- 

 genetically as well as in other respects. 110 



1 . Development of stereome from procambium. 



As the author has shown, isolated groups of bast or collenchyma 

 generally arise from similarly disposed procambial strands (Fig. 

 72 a, b). This rule holds good, not only for subepidermal skeletal 

 strands, but also for bundles which are more deeply situated {e.y. 

 the fibrous strands of Scirpus Holoschoenus, Typha latifolia, Phoenix 

 dactylifera, etc. ; the collenchyma of Salvia officinalis, Cucurbita 

 Pepo, etc.). Where the mechanical system consists of a hollow 

 cylinder which is entirely separated from the mestome-bundles, 

 the antecedent procambial cylinder as a rule likewise originates 

 independently ; this mode of origin is exemplified by the peri- 

 pheral fibrous cylinder of the rhizomes of certain Carices, and by 

 the subcortical bast cylinder of the stem of Cucurbita Pepo (cf. 

 also Fig. 72 c). Not infrecpuently, however, the mechanical system 

 consists of independent strands or of a separate cylinder in the adult 

 state, but nevertheless has a common origin with mestome-bundles 

 in the growing-point. This peculiar method of development is best 

 explained with the aid of one or two concrete instances. In Juncus 

 glaums, each of the principal girders, as already stated, comprises a 

 subepidermal fibrous strand forming the outer flange, and a large 

 semilunar bast-sheath which constitutes the inner flange ; the interval 

 between the two flanges is occupied by the following tissues, considered 

 in centripetal order : a strip of parenchyma, an air-passage, a second 

 parenchymatous zone, a fibrous sheath which serves for local protection, 

 and finally a large mestome-bundle. The whole of this complex array 

 of tissues originates from a single subepidermal procambial strand, which 

 projects far into the fundamental meristem and appears hour-glass- 

 shaped in cross-section (Fig. 5 9 d). Whereas, however, longitudinal 

 division continues for a long time in the innermost and outermost 



