1484 A TEXTBOOK OF THEORETICAL BOTANY 



inner epidermis of the integument, e.g., in Polemoniaceae and Plantaginaceae, 

 is generally the thickened and most protective layer. In Linaceae, however, 

 it is the outer epidermis of the nucellus which forms the protective layer, 

 the integument being thin and weak. 



In seeds produced by bitegminous ovules, the two integuments may 

 remain separate and distinct, or they may become one tissue, or one or other 

 of them may disappear in whole or in part. In the first case the inner testa, 

 which is usually the thinner of the two, has been called the tegmen. The 

 outer layers of the nucellus may also take part in forming the inner covering. 



The histological structure of seed testas is immensely variable and is 

 just as much an expression, though in a small compass, of specific and 

 generic differences as any other part of the plant's structure. We must con- 

 fine ourselves to one or two leading characteristics. There is usually an 

 outer cuticle, which in some seeds may be replaced by, or combined with, 

 a pectic or mucilaginous surface. There is also invariably at least one 

 protective layer of highly thickened cells which may be formed from any 

 layer of either integument. Sometimes there may be two or more thickened 

 layers and the protection they afford is often supplemented by fatty cuticles, 

 either between the integuments or on the inner side of the inner integument. 

 Cutin, indeed, may appear in greater or less amount in the walls of all the 

 cell layers. Commonly, there is also a layer, which may be several cells 

 thick, of thin-walled cells which are filled with reserve food materials. This 

 is called the nutritive layer. The food materials are consumed as the seed 

 ripens and the cells finally collapse into a thin zone of tightly packed cell 

 walls or disappear altogether. 



The thickened protective layers, wherever formed, consist generally of 

 the following cell types, either alone or combined: {a) cells which are either 

 cubical or slightly elongated, with highly thickened and pitted walls; 

 {b) elongated cells of the sclerenchyma type, often closely entwined, with 

 their long axes lying tangentially, while in some families there may be two 

 such fibre layers with the fibre axes crossing at right angles [e.g., Pyrus 

 mains); (c) cells in which the inner and side walls are highly thickened, but 

 the outer walls are thin and cuticularized; (d) narrow cells which are greatly 

 elongated radially, the walls of which are often relatively thin at the inner 

 ends and thicken greatly towards the exterior. The last class of cells are 

 called the Malpighian cells, after their discoverer. They appear polygonal 

 when seen from the seed surface and form a marked palisade layer which is 

 particularly characteristic of larger seeds with thick testas, in such families 

 as the Papilionaceae, the Malvaceae, the Annonaceae and the Cannaceae 

 (Fig. 1358). The palisade may represent the outer epidermis of the outer 

 integument, but it is more usually the hypodermal layer and it may, in fact, 

 be formed from various layers of either the outer or inner integuments. 

 Such a protective layer, in which the cells may be as much as twenty times 

 as long as broad, is an impressive structure and it has been the subject of a 

 good deal of examination. The chief thickening material is secondary 

 "cellulose" or more correctly polyuronide, but tiie outer caps of the cells 



