100 BOTANY PAKT I 



vessels are dead tubes serving for water conduction. They are 

 formed from rows of cells, the lateral Avails of Avhich are peculiarly 

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

 the case, by bordered pits, while the transverse walls become swollen 

 and more or less completely absorbed. In cases where the transverse 

 walls are at right angles to the side walls, they usually become per- 

 forated by a single large round opening while the remains of the 

 wall forms a thickening ring (Fig. 79 C, Fig, 155 g). When the 

 transverse walls are oblique, they are perforated by several openings, 

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

 on this account such vessels are described as scalariform (Fig. 108 q). 

 According to the mode of their wall thickening, vessels are distin- 

 guished as SPIRAL, RETICULATE, or PITTED (cf. also p. 75). When the 

 transversely elongated pits of a vessel are arranged in more or less 

 parallel rows (Fig. 108), 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. 



There is no difference between vasiforra traclieides and vessels other than that 

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

 speaking, vasiform tracheides 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 Gymnosperms the small family of the 

 Gnetaceae only are provided with vessels. In the Angiosperms vessels are 

 only wanting, in some Magnoliaceae {Drimys, TrocJwdcndron). Vessels are not of 

 an unlimited length. A few plants, however, such as the Oak, and especially 

 climbing woody plants, or Lianes, have vessels several metres long ; but, as a 

 rule, their length is not more than a metre, and in plants the woody portion of 

 which conducts water only by vessels, the vessels have an average length of only 

 ten centimetres. The length of an individual vessel is defined by the presence 

 of transverse walls, which are not perforated but bear bordered pits. 



A similar fusion to that seen above to occur in the formation of 

 laticiferous vessels is also met with in fungal hyphae ; by an alisorp- 

 tion of a part of the wall where two branches come into contact, their 

 protoplasmic contents unite. A still more complete fusion is exhibited 

 by the naked amoebae of a Myxomycete in forming the plasmodium 

 (p. 56). The fusion of the sexual cells in the process of fertilisation 

 possesses special characteiistics which place the process in a distinct 

 category (p. 93). 



Tissues (1'"^) 



Origin of Tissues. — A continuous aggregation of cells in intimate 

 union is called a tissue. The origin of vegetable tissues is, in general, 

 attributable to cell division. It is only in the Fungi and Siphoneae 



