150 



BOTANY 



PART I 



and narrow, pointed at both ends, and with thick walls provided with 

 narrow oblique pits. The cells of the storage parenchyma (hp) are 

 rectangular and prismatic or are spindle-shaped ; they are usually 

 elongated in the direction of the long axis and have either thin or 

 thick walls with small, circular, simple pits. They contain abundant 



reserve materials (starch, oil, or sugar). 



Intercellular spaces only occur in the paren- 



chymatous strands. 



In many Leguminosae, in the Willow, Poplar, 

 and species of Ficus, the water-conducting elements 

 of the wood consists of tracheae only. 



The tracheides and wood-fibres are frequently 

 more than 1 mm. in length and are considerably 

 longer than the cambial cells from which they arose. 

 This increased length, like the increased width of 

 the larger tracheae, is attained by sliding growth 

 (p. 48 ; -Fig. 174). In the formation of wood paren- 

 chyma the cambial cells undergo repeated transverse 

 divisions. The resulting parenchyma thus consists 

 of rows of cells, the origin of which from a cambial 

 cell is indicated by the row ending above and below 

 in a pointed cell (Fig. 173 hp}. 



The walls between cells of the wood parenchyma 

 or medullary rays and the vessels have bordered 

 pits on the side towards the vessel only, while the 

 larger pits in the living cell have no borders ; such 

 pits, in contrast to those bordered on both sides, 

 are characterised by the absence of a torus from 

 the pit membrane. The walls separating vessels 

 and wood - fibres and those between the latter and 

 parenchyma cells are, on the other hand, usually 

 without pits. 



In woods composed of vessels, wood-fibres, and 

 parenchyma there are frequently transition forms 

 between the typically - constructed elements, and 

 there is a corresponding lack of sharp distinction 

 as regards function. Narrow tracheae (Fig. 173 tg} 

 lead on to the tracheides (Fig. 173 gt, t). Narrow, 

 sharply-pointed tracheides (fibre tracheides, ft), the 

 function of which is mainly mechanical, form the 

 transition to the wood-fibres (h). Slightly thickened 



wood-fibres which retain their living contents (ef) and are either without or with 

 transverse walls (gli) form the transition to the cells of the wood parenchyma (7^). 



In the wood of Gymnosperms there are only tracheides with 

 typical bordered pits, together with some wood parenchyma and a 

 considerable amount of parenchyma of the medullary rays. The 

 division of labour is here less advanced, the same elements being 

 concerned with the mechanical and water - conducting functions. 



y-^( 



wood-fibres. /, In tangential 

 longitudinal section ; II, in 

 transverse section along the 

 dotted line in J. A, I, II, cells 

 in the young condition ; B, I, II, 

 after sliding growth has taken 

 place. (After ROTHERT.) 



