CRYPTOGAMS AND PHANEROGAMS. 



241 



and some of the cells surrounding it in this group, become dissolved 

 before maturity ; the more external ones, on the other hand, are 

 provided with peculiar thickenings, and form the " fibrous " layer 

 by the aid of which the dehiscence of the anther takes place; 

 (3) an external layer, the epidermis, enclosing all the other layers 

 (Fig. 247). 



In some Conifers (Ciqwessits, Thuja, and several species of Jitniperux) the 

 microsporaugia (pollen-sacs), which are situated on the under side of the stamen, 

 are covered by a thin structure which seems to be a continuation of the lamina 

 and which is supposed to be homologous with the indusium of the Ferns. 



The Ovule in the Phanerogams arises most frequently on a 

 projecting portion of the carpel, termed the placenta. The ovules 

 (compare the sporangium of the Eusporangiatae and especially the 



'TIC 



liC 



FIG. 248. Development of the ovule in the Red Currant, Ribes nibrum, arranged alpha- 

 betically in the order of development. A Is the youngest stage, E the oldest, ii Inner 

 integument; ie outer integument ; we nucellusj in archespore (mother-cell of the euibryo- 

 sac). 



pollen-sac) take their origin from a group of cells which lies beneath 

 the epidermis (Fig. 248 A, JB). First of all a small papilla is 

 formed, which is later on provided with a vascular bundle and 

 becomes the funicle ; this probably has the same value as the 

 projections ("placenta") on which the sori in the Ferns are 

 attached. Only one macro sporangium (nucellus ; Fig. 248 we) 

 is developed at the apex of the funicle. This arises by a process of 

 cell-division exactly corresponding to that by which the pollen- 

 w. B. 



