74 



are transversely elongated and are situated close together, one- 

 above the other, the thickened portion of the wall alternating 

 regularly with the pit apertures, they look like the rounds o a 

 ladder and the vessel is then described as scalariform. 



The essential difference between a vessel and a tracheid 

 consists in the fact that the latter is a single cell and the former 

 . is produced by the fusion of several cells. The tr cheid corre- 

 sponds, as it were, to a single joint of a vessel. Vessels, however, 

 frequently attain a much larger diameter than do tracheids 

 and they are often clearly visible to the naked eye as pores 

 on the transverse section of a stem. Neither tracheids 

 nor vessels contain protoplasm, they are tubes whose special 

 duty is the conveyance of water from the roots to the leaves. 

 Intermediate forms between these typical elements often occur. 



The so-called sieve-tubes are formed, like vessels, by the 

 fusion of a number of cells, but in this case the transverse 

 partitions, instead of being completely absorbed, are penetrated 

 by a number of small holes, such perforated partitions 

 being called sieve-plates. The walls of sieve-tubes are never 

 lignified and are usually thin, with an interior lining of living 

 protoplasm. They are the principal conductors of organic 

 food substances from the leaves to the places where they are 

 required. 



Many plants contain so-called laticiferous -tubes or -vessels. 

 Sometimes these arise by the continued growth and elongation 

 of a single cell and sometimes by the fusion of several cells, as 

 in ordinary vessels. 



Laticiferous tubes are usually branched, their walls have a 

 thin lining of protoplasm and are usually thin, while their sap 

 consists of a milky, usually white, fluid. Such tubes are 

 common in Figs, Euphorbia, and many other plants, and their 

 latex often contains valuable caoutchouc and gutta-percha. 

 The so-called resin-canals of Conifers are really intercellular 

 spaces into which resin is excreted from the neighbouring 

 cells. The oil glands in the leaves of the Orange, Lime and 

 other plants are formed by the disorganisation of a group of 

 cells, the cell walls being absorbed and the cavity so formed 

 -being more or less filled with ethereal oil. 



J y s s s tems. 69. The tissues of the higher 



plants are usually classified in three groups, or systems, as 

 follows : 



(1) Tegumentary Tissue System. 



(2) Vascular Bundle Tissue System. 



(3) Fundamental Tissue System. 



