SECONDARY THICKENING. NORMAL DICOTYLEDONS. 479 



transverse section of which is almost isodiametric, while those of others are ridge- 

 shaped and deeply projecting as .described at p. 156. The former are chiefly vessels, 

 the latter usually tracheides ; but the separation of the two forms is hard to carry out 

 with certainty on account of the difficulty of establishing the presence or absence of 

 the vascular perforations. The species of Opuntia and Cereus have reticulated 

 vessels, which may be accompanied by tracheae with the ridge-shaped thickenings 

 above mentioned. 



The cases described, however, constitute exceptions as compared with the great 

 majority especially of arborescent or shrubby Dicotyledons. In the latter the vessels 

 of the secondary wood are pitted vessels, the wall, apart from the pits, being either 

 smooth, or with a fine spiral fibre on the inner side (comp. Fig. 205). The pits of 

 the vessels are bordered and correspond one with another, at least on those surfaces 

 which border on vessels or tracheides (comp. Sect. 38). On the surfaces bordering 

 on non-equivalent elements, various conditions occur, which will be stated below, 

 chiefly according to Sanio. 



Where pitted vessels are adjacent to sclerenchymatous fibres with unbordered pits 

 (comp. Sect. 143), the pitting may be wholly absent (Olea europsea. Fuchsia globosa 

 according to Sanio) ; in most cases pits are present, though they are always less numerous 

 than those on the surfaces contiguous with tracheae, and they also differ from the latter 

 in form. On the surfaces adjoining fibres with unbordered pits, the pits on the wall of 

 the vessel are bordered, but smaller than those on the boundaries of vessels, in Hedera 

 Helix, Euonymus latifolius, europaeus, and Syringa vulgaris; they are unbordered in 

 Sambucus nigra, racemosa, Acer platanoides, Salix acutifolia, hippophsefolia, Populus 

 pyramidalis, .S^sculus Hippocastanum, Rhamnus Frangula, Aucuba japonica, Pittosporum 

 Tobira. The surfaces adjacent to parenchyma and fibrous cells have sometimes bordered, 

 sometimes unbordered pits, and sometimes both kinds. The first, for example, is the case 

 in Quercus pedunculata, Diospyros virginiana, Juglans regia, Porlieria hygrometrica, 

 Spartium scoparium, Caragana arborescens, Sophora japonica, Acacia Sophora, Morus alba. 

 Daphne Mezereum, Ribes rubrum, Syringa vulgaris, Casuarina equisetifolia. Hibiscus 

 Rosa sinensis, Pseonia Mutan, Ficus Sycomorus, Olea europaea, Nerium Oleander, Tamarix 

 gallica, Punica Granatum, Justicia carnea ; these pits are unbordered in Hedera Helix, 

 Sambucus racemosa, nigra, .Esculus Hippocastanum, Rhamnus Frangula, Syringa Josikaea, 

 Solanum Dulcamara, Populus pyramidalis, Salix hippophaefolia, acutifolia, Vitis vinifera, 

 Magnolia tripetala, acuminata. Hydrangea hortensis; — both kinds occur in Bombax 

 Ceiba, Ficus rubiginosa, Jatropha Manihot, Fuchsia globosa and Eugenia australis. 



As a rule the pits bordering on cells are relatively large ; they are rarely small and 

 very numerous (Hydrangea hortensis) ; where two kinds occur the bordered often differ 

 from the unbordered also in their superficial outline. The pits of the vessel always 

 correspond with the unbordered ones of the non-equivalent elements, and in fact those 

 of the latter are always of the same breadth as the border of the pits on the vessel. 



As an exception we must mention the wall destitute of pits with which the vessels 

 border on the fibrous cells in Punica Granatum. 



Where the lateral walls of the members of a vessel have a spirally thickened inner 

 layer in addition to the pits, the spirals are always present on the surfaces adjacent to 

 tracheae, with one exception to be mentioned at the end ; in certain cases they also exist 

 on the surfaces adjacent to all the other, non-equivalent elements (Tilia parviflora, Pitto- 

 sporum Tobira, Prunus domestica, Laurocerasus). In other cases the spirals are absent 

 on the surfaces which border on parenchyma, while they are present on the others 

 (Amygdalus communis and other Amygdaleae) ; or they are absent where vessels border 

 on one another and on parenchyma, and are only present on the surfaces which are 

 adjacent to fibres. 



