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A TEXTBOOK OF THEORETICAL BOTANY 



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Malvaceae, where the palisade is formed by the inner integument, the 

 light line may be very prominent, though there is practically no suberization. 

 Beneath the palisade layer there may be other sclerotic layers, some- 

 times of stone cells, sometimes of hour-glass-shaped cells, with big inter- 

 spaces, forming an "arcade layer ". 

 '. These cells are called osteosclereids 



from their resemblance to knuckle 



bones. Dark-coloured tannin de- 



"--"^ . posits commonly occur and there 



is often a pigmented layer, which 

 may be a special layer of cells 

 (Lifiiun), or may consist of the caps 

 of the Malpighian cells or of the 

 walls of the outer epidermis. Where 

 the chief protective layer is not 

 superficial there may be one or 

 more layers of relatively thin- 

 walled cells external to it, including 

 the outer epidermis, which is often 

 large celled. In some families the 

 outer epidermis constitutes a swell- 

 ing-layer whose walls are thickened 

 with mucilaginous material that 

 swells readily in water, bursting 

 the cells and forming an enclosing 

 coat of mucilage around the seed. 

 This substance is not chemically 

 uniform. In most cases it con- 

 sists of pectin, but in others it colours blue with iodine and is apparently an 

 amyloid. One of the most familiar cases is that of Linseed {Liniini iisitatis- 

 simiini), whose seeds, soaking in water, look like frog-spawn. Here the 

 mucilage is amyloid, but in Cistaceae it is generally pectin, except in some 

 species of HeUanthemum. Perhaps the most striking examples are among 

 the Polemoniaceae {Gilia, Cobaea, Colloniia), where the thick slime layers 

 on the epidermal walls include spiral or ring-like bands. Contact with 

 water causes the amyloid material to swell enormously, bursting the tops 

 off the cells and protruding in long columns, in which the unswollen spirals 

 or rings are extended (Fig. 1359)- 



We have already referred to certain plants {e.g., Crimim) in which there 

 is no testa and the endosperm forms the outer layer of the seed. Many 

 other types have much reduced or else rudimentary testas, e.g., Veronica 

 hederaefoUa, whose peculiar incurved seeds have only a single layer of very 

 small cells covering the endosperm, and these soon disrupt, so that only 

 their inner walls remain, which may however have a high protective quality. 

 Similarly, many minute seeds have testas consisting only of a single cell 

 layer, e.g., Ericaceae. The bitegminous seeds oi Asparagus depend chiefly 



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Fig. 1359. — Colloniia grandiflora. Section of 

 the testa after wetting showing the ex- 

 truded spirals in the mucilage layer 

 derived from the exploded epidermal 

 cells. 



