32.] CHAPTER II. THE TISSUES. 169 



Hydrocotylevulgaris, Bupleurum fruticosum and other Umbelliferse) ; 

 or, in consequence of the presence of both secretory ducts and of 

 fibres (e.g. Ligulifloral and Tubulifloral Composites.) 



A remarkable form of conjunctive tissue is that which invests the two vascular 

 bundles in the acicular leaves of Pinus, and, to a less degree, of otber Conifers. 

 The tissue consists of parenchyma with some fibrous sclerenchyma ; in the 

 parenchyma two special kinds of cells can be distinguished, which constitute 

 what is sometimes termed the transfusion-tissue ; namely, cells with unlignified 

 and unpitted walls, distinguished by their abundant protoplasmic and proteid 

 contents ; tracheidal cells with slightly lignified walls and bordered pits, without 

 protoplasmic contents ; the former may be regarded as an extension of the 

 sie/e-tissue of the bundle, the latter as an extension of the tracheal tissue. 



The fibres (stereom) of the pericycle are distributed in various ways. They 

 may either form a continuous ring, separated from the phloem of the vascular 

 bundles by several layers of parenchymatous pericycle-cells (e.g. Berberis, 

 Cucurbitaceae, CaryophyllaceaB, Lonicereae, Aristolochiaceae (Fig. 130), and many 

 Monocotyledons, especially in Gramme, Cyperaceae, and Juncaceaa; or the ring 

 may be in direct contact with the vascular bundles (many Dicotyledons, e.g. 

 Hypochceris radicata, Podophyllum, Plantago, etc. ; many Monocotyledons, e.g. 

 Ilestiaceae, Liliaceae, Smilaceae, Iridaceae, Orchidaceae, Typhaceae, Tamus, etc. 

 [see Fig. 124J5]); or the fibres may form strands opposite to the phloem of 

 each bundle, either abutting directly on the phloem, or separated from it by 

 one or more layers of parenchymatous pericycle-cells (e.g. among Dicotyledons, 

 various Composite, Labiatae, Legumiuosae, Ranunculaceas, etc. ; exactly this 

 arrangement does not occur in Monocotyledons) ; or the fibres may be scattered 

 either singly or in groups, without any relation to the vascular bundles (e.g. 

 many Solanaceae, Iberis sempervirens, species of Viburnum, Fraxinus dimorpha, 

 Ligustrum vatyare, Apocynaceae, Asclepiadaceae, Malvacea?, etc). 



The Pith (or medulla) consists, typically, of parenchymatous 

 cells with thin walls and protoplasmic contents ; but in many 

 cases sclerenchyma is differentiated in it. 



The most important fact with regard to the parenchyma of the 

 pith is that, in many cases, the cells forming the central portion 

 of the pith soon die, or even the whole of them (e.g. Elder). When 

 this is the case, the dead cell- walls frequently undergo disorganisa- 

 tion, so that the stem becomes hollow. 



The sclerenchyma of the pith may consist of scattered strands 

 (e.g. stems of some Palms, such as Cocos, Astrocaryum, Leo- 

 poldinia) ; or it may form a ring connecting the inner ends of 

 the bundles of the hollow vascular cylinder (e.g. Bougainvillea 

 spectabilis, woody PiperaceaB such as Artanthe and Chavica). 



The bulk of the pith varies very much. It is relatively very 

 large in tuberous shoots, such as the Potato, Apios, etc. 



