MORPHOLOGY 



they usually become perforated by a single round opening while the 

 rest of the wall remains as a thickening ring (Fig. 86, C). When the 

 transverse walls are oblique, they are then perforated by several open- 

 ings, between which portions of the wall remain, like rungs of a ladder 

 (Fig. 94, q). According to the mode of their wall thickening, vessels 

 are distinguished as spiral, reticulate, or pitted. When the trans- 

 versely-elongated pits of a vessel are arranged in more or less parallel 

 rows (Fig. 94), it is called a scalariform vessel. The thickening of 

 the vessel walls is always lignified. The living contents of the cells, 



Fig. 92.— Parts of sieve-tabes of CucurMta Pepo, hardened in alcohol. A, Surface view of a sieve- 

 plate ; B, C, longitudinal sections, showing segments of sieve-tubes ; D, contents of two sieve- 

 tube segments, after treatment with sulphuric acid ; s, companion cells ; u, albuminous con- 

 tents ; pr, peripheral cytoplasm ; c, callus-plate ; c*, small, lateral sieve-pit, with callus-plate, 

 (x 540.) 



after the perforation of the transverse walls, become completely 

 absorbed, and the fully-formed vessels or trachea contain only water 

 and a limited amount of air. 



There is no difference between vasiform tracheitis and vessels other than that 

 the former are single elongated cells, and the latter fused cell rows. Generally 

 speaking, tracheids are formed in parts of plants still in process of elongation, 

 vessels in parts where growth in length has already ceased. True vessels make 

 their first appearance in some of the Ferns, for instance, in the common Bracken 

 (Pteris aquilina). In the main, despite the name Vascular Cryptogams, Ferns 

 have only vasiform tracheids. Even in the Gymnosperms the Gnetaceae are the 

 only family regularly provided with vessels. It is in the Angiosperms that vessels 

 first become of frequent occurrence. Vessels are not of an unlimited length. A 

 few plants however, such as the Oak, and especially climbing woody plants, as the 

 Lianes, have vessels several metres long ; but, as a rule, their length is not more 



