CYCADOFILICALES 



33 



each chamber being traversed longitudinally by a single vascular 

 strand, the strands being connected with the main bundle that enters 

 the chalaza. The pollen chamber is peculiar in that it is not the usual 

 open cavity in the beak of the nucellus, but is a cylindrical (bell- 

 shaped) crevice between the epidermis of the free tip of the nucellus 

 and its central core of tissue (fig. 27). The archegonia probably 

 occurred in a ring be- 



neath this crevice, 

 preparations 



Our 

 of L. 

 ovoides from Lomax show 

 the method of formation 

 of this peculiar pollen 

 chamber (figs. 28 and 

 29). The free beak of 

 the nucellus is covered 

 by an epidermal layer 

 whose cells have thick, 

 resistant walls. Within 

 this layer, near the base 

 of the beak, the thin- 

 walled tissue begins to 

 break down; and this 

 breaking down extends 

 upward beneath the epi- 

 dermis to the apex of the 

 beak, resulting in the 

 characteristic crevice-like 

 pollen chamber surround- 

 ing a central core of tissue. A diagrammatic longitudinal section of 

 the seed is shown in fig. 30; and diagrammatic transverse sections at 

 various levels through the pollen chamber, in figs. 31-34. The contrast 

 of this Lagenostoma method of pollen chamber-formation with the ordi- 

 nary method, such as occurs in the Stephanospermum type of seeds (see 

 below), in the cycads, etc., is striking. In the latter case the pollen 

 chamber begins to develop at the tip of the nucellus and is central; 

 while in Lagenostoma it begins to develop at the base of the nucellar 

 beak and is peripheral. It must be understood, therefore, that the 



Fig. 28. — Lagenostoma ovoides: longitudinal 

 section of a seed; X12. — From photomicrograph 

 by Land. 



