33.] CHAPTER II. THE TISSUES. 173 



trenching on the pith (e.g. many Ranunculaceee, such as Actaea, 

 Cimicif uga, Thalictrum, Podophyllum, Diphylleia ; Nymphseaceae ; 

 Monocotyledons generally). A good example of this is afforded 

 by a Palm stem (Fig. 132 A). The median leaf- traces first tend 

 toward the centre of the stem ; they then bend outward, thinning 

 out gradually as they descend, and coalesce with the lateral 

 bundles, which do not penetrate so deeply, in the pericycle at a 

 point much lower down. Furthermore, each bundle is somewhat 

 twisted in its course, so that the lower end lies toward a different 

 side of the stem from that on which it entered it. In these 

 cases, when there is a well-defined external ring, the more internal 

 bundles are termed medullary bundles. 



In some plants a number of independent phloem-bundles are developed in the 

 outer part of the pith of the stem, just within the ring of normal vascular 

 bundles (e.g. Solanaceee, Convolvulaceae, Gentianaceae, Apocynaceae, Asclepiad- 

 acese, some Tubulifloral Compositae, CampanulaceaB, etc.), which are termed 

 medullary phloem-bundles. 



Stems with cauline bundles may be monostelic or polystelic ; 

 monostelic stems with cauline bundles are generally gamodesmic 

 having a solid vascular cylinder (e.g. Lycopodium among Pterido- 

 phyta ; some aquatic Dicotyledons, Utricularia, Aldrovanda, 

 Callitriche, Myriophyllum, Ceratophyllum ; some aquatic Mono- 

 cotyledons, Elodea canadensis, Hydrilla verticillata, ISTaias ; and the 

 saprophytic Orchidaceous plants Epipogum Gmelini and Oorallorhiza 

 innata) ; polystelic stems with cauline bundles (e.g. most Ferns, 

 Hydropterideee, Selaginella, among Pteridophyta) are generally 

 gamostelic, the steles forming a network, which can be isolated 

 as a skeleton of vascular tissue, each mesh of which corresponds 

 to the insertion of a leaf. 



The leaf-traces of stems with common bundles, as also the leaf- 

 bundles in stems with a cauline vascular tissue-system, do not 

 always at once join the stele, but may remain isolated for a longer 

 or shorter distance, constituting in fact meristeles (see p. 153) 

 in the cortical region of the stem, or, as they are more commonly 

 called, cortical bundles (e.g. Casuarina ; species of Begonia ; some 

 Ferns, such as Pteris, Aspidium, etc.) 



The relative position of the phloem and of the xylem in a con- 

 joint bundle is subject to some variation ; they may either be 

 side by side, when the bundle is said to be collateral ; or the one 

 may more or less completely invest and surround the other, when 

 the bundle is said to be concentric. 



