72 



BOTANY 



become stellate ; if these points are less uniformly arranged their 

 outline is correspondingly unsymmetrical. In consequence of 

 energetic growth in length, fibre-like, pointed cells are developed. 

 If the walls of such cells become much thickened, they are called 

 SCLERENCHYMA fibres (Fig. 78 A}. These show diagonal markings, 

 due to their elongated pits, which are generally but few in number. 



When fully developed, the living contents 



A R L f suc h ce ^ s are sma U i amount and 



frequently they contain only air. In the 

 last case, they merely act as mechanical 

 elements (stereides) and contribute to the 

 rigidity of the plant as a whole. Cells 

 somewhat similar, but shorter and consider- 

 ably wider, not pointed at the ends, and 

 provided with bordered pits, are called 

 TRACHEIDES (Fig. 78 B}. The tracheides, 

 in their fully developed condition, never 

 have any living contents, but serve as 

 water-carriers for the plant. So long as 

 they remain active, they contain only water 

 and isolated air -bubbles; their active 

 functions afterwards cease, and they become 

 filled with air. Tracheides which are speci- 

 ally elongated, and at the same time have 

 only a narrow lumen, and, like the scleren- 

 chymatous fibres, serve merely mechanical 

 purposes, are known as FIBRE TRACHEIDES. 

 Very long tracheides with a wide lumen 

 and thin walls, functioning, like typical 

 tracheids, as water-carriers, are distin- 

 guished as vasiform or VASCULAR TRACHE- 

 IDES (Fig. 78 C). They are characterised by 



Fio. 78. .-I, A sclerenchyinatous ,1 i i i' i i 



tibre: js.atracheid: c, part of the annular, spiral, or reticulate markings 



a spiral tracheid ; D, part of a of their thickening layers, and may also be 



latex tube. (A, K, c, x loo ; D, pr0 vided with bordered pits. The walls of 



tracheides are always lignified, while those 



of the sclerenchyma fibres may or may not have undergone this change. 

 Of all the cells in the more highly organised plants, the LATEX 

 CELLS or milk cells, also spoken of as latex tubes, attain the greatest 

 length. In the Euphorbiaceae, Moraceae, Apocynaceae, and Ascle- 

 piadaceae they arise from cells which are already differentiated in the 

 embryo. Growing as the embryo grows, they branch with it and 

 penetrate all its members, and may thus ultimately become many 

 metres long. The latex cells themselves have, for the most part, 

 unthickened, smooth, elastic Avails which give a cellulose reaction. 

 They are provided ( 01 ) with a peripheral layer of living cytoplasm 



