MORPHOLOGY 



95 



mucilaginous sap contains albumen, starch, glucose, and tannins in 

 addition to inorganic compounds. 



A cell fusion also takes place in the formation of VESSELS or 

 TRACHEA, but it should not be considered as a union between living 

 cell bodies, but merely as one between cell cavities. The mature 

 vessels are dead tubes serving for water conduction. They are 

 formed from rows of cells, the lateral walls of which are peculiarly 

 marked by spiral or reticulate thickenings, or, as is more frequently 

 the case, by bordered pits, while the transverse walls become more or 

 less completely absorbed. In cases where the transverse walls are 



FIG. 103. Parts of sieve-tubes of Cucurbita Ptpo, hardened in alcohol. A, Surface view of a sieve- 

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

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

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

 ( X 540.) 



at right angles to the side walls, they usually become perforated by 

 a single large round opening while the remains of the wall forms a 

 thickening ring (Fig. 73 C). When the transverse walls are oblique, 

 they are perforated by several openings, between which portions 

 of the wall remain, like rungs of a ladder (Fig. 105 q). According 

 to the mode of their wall thickening, vessels are distinguished as 

 SPIRAL, RETICULATE, or PITTED. When the transversely elongated 

 pits of a vessel are arranged in more or less parallel rows (Fig. 105), 

 it is called a SCALARIFORM VESSEL. The thickening of the vessel 

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

 perforation of the transverse walls, become completely absorbed, and 

 the fully formed vessels or tracheae contain only water and a limited 

 amount of air. 



