THE ANGIOSPERMAE 



1383 



of the outer integument, which turns backwards over the surface of the 

 ovule, which it partially envelops. This type of outgrowth is called a 

 caruncle. 



During the development of the embryo sac, the integumental tissues 

 may show partial dissolution or diiferentiation. The most noteworthy 

 development is that of an ovular tapetum, or endothelium, formed by the 

 innermost integumental cell-layer. This is characteristic of ovules in which 

 the nucellus disorganizes, i.e., principally in the Metachlamydeae, so that 

 the tapetum immediately surrounds the embryo sac (Fig. 1285). The cells 

 possess dense contents and large nuclei and are sometimes binucleate. 



Fig. 1285. — Calceolaria mexicaua. A, B, and C, Successive stages in the degeneration of the 

 nucelkis and development of integumentary endotheHum in a tenuinucellate ovule. 

 {After Srinath.) 



They are often radially elongated and, in the Compositae, may be more than 

 one-layered. 



The function of this tapetal layer has been much discussed. The cells 

 have the characteristics of secretory cells and the most obvious suggestion 

 is that they serve to nourish the embryo sac, as the anther tapetum nourishes 

 the pollen grains. The chief difficulty in this opinion is the presence of a 

 cuticle between the tapetum and the embryo sac. This has been demon- 

 strated in a large number of Metachlamydeae, but it is not certain that it 

 forms a continuous barrier. Srinath has indeed suggested that it is not a 

 true cuticle but simply the remains of the nucellus. If there is no effective 

 barrier, it is easy to suppose that the tapetum digests materials from the dis- 

 organizing middle lavers of the integument and passes them on to the 

 embryo sac. An alternative theory of its function is that it acts as a restrain- 

 ing layer, protecting the embryo sac from rupture during the growth of the 



