THE ANGIOSPERMAE 1389 



outer integument only; most commonly it is bordered by both integuments, 

 where two are present. In such cases the micropyle may have two distin- 

 guishable parts, the exostome and the endostome, which may be different 

 in size and may sometimes not be in the same line, so that the whole passage 

 is crooked. The two portions may even be at right angles, as in some Legu- 

 minosae. Extreme cases occur where the micropyle is completely closed by 

 the contact of the integumental margins, or even obliterated, as in the 

 Rubiacean genus Houstonia mentioned above, where only one massive in- 

 tegument is present. In a few species the tip of one of the integuments is 

 extended into a tuft of filaments which grow up towards or into the stylar 

 canal and form part of the conducting tissue for the pollen tubes. Examples 

 are: Isolepis gracilis (Cyperaceae) where the filaments come from the outer 

 integument, and Myriocarpa longipes (Urticaceae) where they come from 

 the inner integument (see p. 1440). We have already referred (p. 1234) to the 

 obturators which in some plants occlude the micropyle. They are usually 

 outgrowths from the placenta or from the base of the stylar canal and they 

 are in some respects the converse of the conducting filaments growing 

 from the integument, as the obturator also functions as part of the con- 

 ducting tissue system, linking the ovule and the stigma. Sometimes, as in 

 Grevillea, the micropyle may be filled with a mucilaginous secretion, which 

 comes partly from the integument and partly from the apex of the nucellus. 

 This may perhaps facilitate penetration by the pollen tube. 



There is no record of any endothelial layer lining the micropyle, even 

 in species with massive integuments. The lining cells are apparently not 

 specially modified. 



The posture of the completed ovule presents a number of variations. 

 The simplest of these is the straight position, the ovule standing upright from 

 the placental surface with the funicle and the nucellus in the same straight 

 line. This is the orthotropous position. It is relatively uncommon, the 

 Polygonaceae providing the best known examples. The commonest form 

 of ovule is that in which the funicle is lengthened and the whole body of the 

 ovule is inverted through 1 80 ' , with the micropyle close to the placenta. The 

 nucellus remains straight. This is the anatropous position (Fig. 1290). 



There are so many variations of ovular posture that sharply defined 

 classes cannot be separated, but in addition to the above the following types 

 are sufficiently well-marked to have received names, (i) Hemitropous. The 

 ovule is turned through 90' so that it makes a right angle with the funicle, 

 which is attached to it at the middle of its long axis. The nucellus and 

 embryo sac are straight. (2) Campylotropous. Like the last, but the apex 

 of the ovule is bent over into the anatropous position. Nucellus and embryo 

 sac are therefore bent through a right angle or more. (3) Amphitropous. 

 Externally resembles the anatropous posture, but the inversion bending has 

 taken place in the body of the ovule itself, the basal half remaining ortho- 

 tropous and the apical half being anatropous. The nucellus and embryo 

 sac are thus bent into a semi-circle. Within this semi-circle is enclosed a 

 tissue which may be either of nucellar or of integumentary origin, which 



