212 MANUAL OP BOTANY 



placenta with its ovules as having a similar morphological value. 

 In the anther the sorus forms a single body, or synangium, while 

 in the pistil the separate ovules or megasporangia are free. 

 Some botanists, indeed, prefer to look upon the placenta as 

 bearing a number of sori instead of corresponding to a single 

 one. In their view each ovule corresponds to a sorus of Azolla, 

 which contains a single megasporangium ; and the indusium of 

 the latter is represented by the integuments of the ovule. The 

 latter may, however, be a special protective coating for the ovule 

 without being comparable to an indusium, from which indeed 

 they differ in being developed from the body of the ^orangium, 

 and not from the surface of the sporophyll. 



An ovule consists of an ovoid body, which in section shows 

 the parts represented in fig. 469. There is a central mass of 

 tissue which constitutes the bulk of the structure, and is called 

 the niicellus. This is surrounded by certain coats or integu- 

 ments which grow up round it and enclose it, with the exception 

 of a small canal or passage at the apex. This passage is known 

 as the microiiyle. In the interior of the nucellus is a conspi- 

 cuously large cell with thin walls, which is the viegaspore or 

 emhryo-sac. The megaspore never lies free in the interior of 

 the sporangium, as do the microspores already described. It 

 is always from the first surrounded closely by the tissue of 

 the nucellus, and in its growth it absorbs the whole or a good 

 deal of the latter : in some cases encroaching even upon the 

 substance of the integuments. Sometimes more than one 

 megaspore is produced, but this is unusual. Only one comes 

 to maturity in such cases. 



The ovule is either attached directly to the placenta, when 

 it is said to be sessile {fig. 436, gr ) ; or Indirectly by a stalk 

 called the funiculus or fanicle (figs. 418, ov, and 440), when 

 it is described as stalked. The point of attachment of the ovule 

 to the placenta if sessile, or to the funiculus when stalked, is 

 termed the hilum. 



Some ovules, as those of the Mistletoe (fig. 466), consist 

 simply of the nucellus, n, and its megaspore, c, as above de- 

 scribed, in which case the nucellus is termed naked (fig. 465) ; 

 but in almost all plants it becomes enclosed in one or two 

 coats. In the Walnut there is but one coat, which appears at 

 first as a little circular process around its base ; this gradually 

 increases in size, and by growing upwards ultimately forms a 

 sheath or cellular coat to the nucellus, which it entirely closes 

 except at the apex, where a small opening maj be always 



