1384 A TEXTBOOK OF THEORETICAL BOTANY 



endosperm, or else protecting it from loss of nutritive materials by exuda- 

 tion. This view does not seem to be well founded, but further inquiry is 



needed. 



The use of the name tapetum for this layer is objectionable because the 

 tissue is in no way homologous with the tapetum in the anther. The 

 latter is derived from the archesporium, which, of course, is not true of these 

 integumental cells in the ovule. 



Little enough is known about the functions of the integuments in the 

 early stages of the ovule. Chlorophyll occurs in the outer integument in 

 several Monocotyledons {Lilium, Gladiolus, Amaryllis, Nerine) and stomata 

 have also been found in a variety of genera {Canna, Nelumbiiim, Nerine, 

 Aquilegia) and are probably to be found elsewhere. Deposits of starch are 

 fairly common and storage seems to be increased at the beginning of endo- 

 sperm formation. Buell has recorded in Dianthus chinensis that starch accu- 

 mulates in the integuments around the micropyle and in the nucellus around 

 the embryo sac and in a track up to the nucellar apex below the micropyle. 

 After fertilization all this disappears and a secondary deposit appears at the 

 base of the ovule, probably related to nutrition during endosperm formation. 



Although the inner integument is in the closest contact with the nucellus, 

 it is not united to it and is usually separated by a cuticle. Each ovule has 

 normally three cuticles, one externally, secondly a double layer between 

 the integuments and thirdly another double layer which belongs partly to 

 the inner integument and partly to the nucellus, which usually has a delicate 

 cuticle. Assuming that the cuticle is a relatively impermeable membrane, 

 this implies that the embryo sac must draw its nourishment from the basal 

 region of the ovule, rather than directly from the integuments. Only in 

 certain families, such as the Rubiaceae, where the nucellus undergoes great 

 reduction, is there an organic fusion between it and the inner integument. 



The vascular supply to the ovule enters the funicle from the placenta 

 and normally ends in the basal part of the ovule, the chalaza. It is some- 

 times an organized vascular bundle but often consists of little more than a 

 procambial strand of elongated cells. Only rarely does the bundle continue 

 into the integuments, but where it does it may branch into several strands, 

 disposed around the ovule, as is the case in Myrica in which the integuments 

 are almost completely free from the nucellus. Such integumentary bundles 

 are frequent in the Gymnosperms and it has been maintained that 

 their presence in Angiosperms is a primitive character. They do occur in 

 certain families which have claims to be called primitive, i.e., Ranunculaceae, 

 Magnoliaceae, Betulaceae and Myricaceae, but they are also known in 

 members of a number of truly advanced families, such as Cuscutaceae, 

 Caprifoliaceae and Compositae, which largely discounts the idea of their 

 phylogenetic significance (Fig. 1286). 



The nucellus in most Archichlamydeae and Monocotyledons is a 

 massive tissue, which persists into the ripening seed, where, in a few families 

 such as the Piperaceae, it may even form a special nutrient tissue, the peri- 

 sperm. Such a condition is called crassinucellate. On the other hand the 



