PRIMARY PARENCHVMA. PITH. 403 



stitute the zone distinguished as the medullary sheath, to which we shall have to return 

 in Chap. XIV. In like manner the parenchyma of the cyHnder of bundles in Mono- 

 cotyledons becomes as a rule narrower-celled towards the outer surface of the latter. 

 In those stems of Dicotyledons and Equiseta which become hollow, the narrower 

 peripheral elements of the pith are persistent ; the case is the same in the IVIonocoty- 

 ledons for the elements of the entire zone which surrounds the (javity and contains 

 the bundles. 



According to the comparative investigations of A, Gris\ the pith of Dicotyledonous 

 ligneous plants at first consists entirely of parenchyma, in which crystal-sacs appear con- 

 stantly, and sclerenchymatous elements not unfrequently. On the complete development 

 of the one-year-old stem or branch, the cells of the parenchyma sometimes become 

 emptied of their contents, and then dry up, so as to contain air (empty cells, ' cellules 

 inertes ' of Gris) ; sometimes they remain as active parenchymatous cells (cellules actives), 

 which alternately store up and give back products of assimilation, especially starch and 

 tannin, according to the periods of vegetation. According to Gris, this activity continues 

 for years ; in the case of Platanus occidentalis, Gleditschia ferox, Betula alba, Quercus 

 Robur, and Fraxinus, it has been traced as far as the 20th year of life. The active cells 

 are, as a rule, distinguished from the empty ones by their smaller size, and their thicker, 

 finely-pitted walls. 



Only in a few woods does the pith become entirely empty and dried up: Sambucus 

 nigra. In most cases it either consists of active cells, with crystal-sacs which are isolated 

 or distributed in larger groups, or of these elements and empty cells in varying arrange- 

 ment. The former is termed by Gris homogeneous pith. This occurs with a relatively 

 small number of isolated crystal-sacs in very numerous ligneous plants ; as in the investi- 

 gated species of Pyrus, Cydonia, Aronia, Quercus, Fagus, Betula, Alnus, Platanus, Ilex, 

 Prinos, Buxus, and many EricSceae ; it is interrupted by larger scattered groups of thin- 

 walled crystal-sacs, which are isolated, or arranged in a reticulate manner as seen in cross- 

 section, in Pernettya, species of Rhododendron, Calluna vulgaris, Andromeda polifolia, 

 Cladothamnus, &c. 



Pith, which is compounded of active and empty cells, is called by Gris heterogeneous. 

 It is composed either of an empty central and an active peripheral portion, as in Lonicera 

 fragrantissima, Abelia rupestris, Symphoricarpus vulgaris, Ligustrum, Ornus, Syringa vul- 

 garis, Berberis vulgaris, Ulmus campestris, Celtis, and Rhamnus sp. ; or this arrangement 

 is complicated by the occurrence of active bands in the central empty portion: Pyrus 

 Malus, Sorbus Aucuparia, Aria torminalis, Crataegus Oxyacantha, Amorpha glabra ; or it 

 is everywhere made up of alternating bands of empty and active tissue, in which case the 

 latter chiefly forms longitudinal rows : Viburnum Tinus, and Lantana ; or bands anasto- 

 mosing in all directions in a reticulate manner : Rubus, Rosa "^y and Clethra ; or diaphragm- 

 like transverse plates: Magnolia, and Liriodendron. In the nodes and at the boundary 

 of successive annual shoots, the pith is on the whole tougher, and in the heterogeneous 

 forms has a larger proportion of active elements. 



With regard to further details, for which reference is to be made to Gris, a great con- 

 stancy of structure exists for each species. The examples mentioned of Rosiflorae and 

 Ericacese show that species of like habit, belonging to closely-allied genera of a family, 

 may behave differently. 



In the parenchyma of the primary medullary rays essentially the same conditions 



^ Sur la moelle des plantes ligneuses, Ann. Sci. Nat. 5 ser. torn. XIV. p. 26, pi. 4-7. — Nonvelles 

 Archives du Museum d'Hist. Nat. VI. p. 201. 



^ Compare Mohl, Poren d. Pflanzen-Zellgewebes, p. 27, figs. 27, 37, 38. 



D d 2 



