OUTER CELLS AND TISSUES. 37 



the assimilation of carbon dioxide (during the daytime) ; but 

 the palisade cells are far more powerful than those of the spongy 

 parenchyma as assimilators of carbon dioxide, and this by reason 

 of the large amount of chlorophyll they possess. 



In the centric leaf of Pinus each cell of the spongy parenchyma 

 has curious infoldings of the cell-wall, which, in the adult cell, 

 are known as trabeculee. These increase the available transpir- 

 ing surface of the cell (see Fig. 10, m). 



In bifacial leaves the palisade cells are arranged in groups 

 which converge by their bases on to single cells of the spongy 

 parenchyma, known as collecting cells ; this arrangement facili- 

 tates the diffusion of sugar formed in the palisade parenchyma 

 into the other cells of the mesophyll (see also Chap. x.). 



0. CELLS OF CERTAIN SUPPORTING AND PROTECTIVE TISSUES 

 OCCURRING IN PLANTS IN VAI;I(.)US POSITIONS. 



Under this heading will be described : — 



(a) Cork. 



(h) CoUenchyma. 



(c) .Sclerenchynia. 



These tissues are found in varied positions in the stem, root, 

 or leaf of higher plants, chiefly the Dicotyledons, Monocotyledons, 

 Coniferse, and higher Ferns ; and in all cases their function is to 

 confer elasticity and rigidity,* and act as a means of protection 

 to more delicate tissues. 



(a) Cork is found in the form of layers of varying thickness in 

 the stem or root ; in the latter it is produced from a zone of 

 actively dividing cells known as the pericycle, which occur just 

 internal to the endodermis in those roots which possess a well- 

 marked central cylinder. 



Cork-cells may arise from epidermal-cells, in stems, and in this 

 case are cut off in the first instance from the inner portions of 

 these cells ; but, as a rule, the first division going to produce the 

 cork-forming layer occurs in the first layer of cortical cells just 

 below the epidermis. 



If a longitudinal section be taken of a young stem of Sambucus 



* Rigidity in succulent plants is greatly aided by turgidity of the 

 living cells. 



