278 TISSUE ELEMENTS OF SEED PLANTS 



opposite pits of adjacent cells then form a lens-shaped 

 structure on the cell wall. The extreme example of 

 this type of bordered pit is seen in the tracheids (water- 

 conducting cells) of conifers, f orinstance the commonpine. 



The fine threads of cytoplasm, originally the achro- 

 matic spindle threads (see p. 106), connecting the cyto- 

 plasm of adjacent cells, are sometimes, though not always, 

 confined to the- pit membranes of thick-walled cells. 

 The existence of pits greatly facilitates the passage 

 of substances in solution from one thick-walled cell to 

 another, and through the pits, no doubt, the sugar 

 which supplies the carbohydrate material of which the 

 thick cell wall is made mainly enters the cell. We do 

 not know what determines the formation of a pit at 

 any given spot on the cell wall, though in some cases 

 it is clearly connected with the presence of a bundle 

 of cytoplasmic threads passing through the middle 

 lamella which in some way must arrest the deposition 

 of cellulose at that spot. The formation of thick walls 

 in general is an expression of excess of soluble carbo- 

 hydrate substance in the cell, which is condensed and 

 added to the wall as cellulose. In many cases, indeed, 

 thick-walled tissue is of no particular use to the plant, 

 but is simply formed as a result of carbohydrate excess, 

 when photosynthesis is unchecked, but there is not a 

 sufficient supply of salts to form proteins for the 

 manufacture of new protoplasm. Thus most plants 

 living in dry climates form great masses of thick-walled 

 tissue. But thick-walled cells may be of great use to 

 the plant, since they lend rigidity and toughness to 

 its body {mechanical tissue). 



Local Thickening of Cell Walls. — Sometimes the 

 thickening of a cell wall is not uniform over the whole 

 surface, but local : for instance in a box-shaped cell 



