R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 241 



direction of the spirals is most frequently to the left, and their 

 number in each cell is variable, while the transition from annular to 

 spiral thickening is not unfrequent even in the same vessel. 



Tubular cells with net-like thickening are best seen in succulent 

 herbaceous plants, such as Balsam, Cucumber, and Celandine, and 

 they lie more external than the spiral cells, 



Porose tubular cells (Pitted tissue or Bothrenchyma) are widely 

 distributed, the pores being bordered, and circular or slit-like. 

 Round-bordered porose tubular cells form the greater part of the 

 vessels in Dicotyledons. When the border is extended consider- 

 ably in a transverse direction, we have the so-called scalariform 

 vessels, familiar to you all in the vascular bundles of ferns, and also 

 seen in the stems of the Vine, Balsam, Viburnum, Daphne, &c. 

 The thin transverse partitions are perforated by a single pore, or by 

 net-like or scalariform apertures ; mixed spiral and porose tubular 

 cells are seen in the Lime and Honeysuckle. 



Porose tubular cells with cribriform thickening (Lattice cells) 

 pertain only to the bast poi'tion of the vascular bundle, and tc this 

 group also belong the milk vessels, which carry the coloured sap or 

 latex in many plants. The transverse partitions open by a single 

 large pore, the covering membrane of which is not resorbed, but 

 becomes perforated by roundish or angular apertures, so as to re- 

 semble a sieve or grating, as we see in the Hop, Bryony, Dahlia, 

 Eqiiisetum, &c. ; or when the partition is much inclined, the per- 

 forations are lattice-like, as in Pteris. Tilia, CEsculus, &c. 



Complete resorption of the transverse partitions has taken place 

 in the branched anastomozing tubular cells which constitute the 

 milk vessels, so general in composite plants, Euphorhiacece and 

 Asclepiadacece. The cellulose case of their longitudinal walls is 

 usually weak, but strong thickening layers are seen in those of the 

 tropical Euphorbias. The thin places consist of large pores, the 

 partition wall of which is, by resorption of the closing membrane, 

 variously perforated by smaller pores. The cribrose cells are en- 

 closed by the fibre cells, and equal them in length, attaining their 

 greatest dimensions in the milk vessels of Apocynece and EupJwr- 

 biacecB, on account of the union of several cells into one tube. 

 Lignification takes place but to a slight degree, so that Iodine 

 always gives the reaction of cellulose, and, to observe the milk 

 vessels, long maceration is necessary until they become isolated by 

 decay of the softer tissues around them. 



