CONIFER ALES 323 



out the free nuclear period are simultaneous (fig. 3595). Here 

 again, a figure taken from Land's splendid work on Ephedra, will 

 also illustrate what takes place in conifers. 



A free nuclear period is not confined to conifers, but is likely to 

 occur in any plant where the cell is large in proportion to the nuclear 

 figure. Where the cell is large and the nuclear divisions follow in 

 rapid succession, one division follows another before a wall can be 

 formed on the spindle. As the number of nuclei increases, and the 

 interval between mitoses becomes more prolonged, walls make their 

 appearance. 



The extent of the free nuclear period depends upon the size and 

 shape of the megaspore outhne. If it is long and narrow, the period 

 will be shorter; if more or less spherical, the period will be longer. So 

 there may be only dozens of nuclei when walls begin to come in; 

 or there may be hundreds. Jager^^s estimated the number in 

 Taxus haccata at 256, and Dupler'^^ found the same number in 

 T. canadense. Coulter and Land'S2 ^X^q found 256 in Torreya 

 taxifolia, a number already reported for Zamia and Ginkgo. This 

 must be regarded as a very low number for a gymnosperm. 



Dr. Margaret Ferguson''^^ finds a much larger number in Pinus 

 strohus. She counted 2,000 when walls were coming in. Since these 

 mitoses are simultaneous, that would mean 1,000 free nuclei. Nor- 

 EN''*^ estimated the number in Juniperus communis as about the 

 same. 



Period of wall formation. — There are two types of wall formation 

 in the female gametophy te, one of which may be illustrated by Pinus 

 and the other by Taxus. 



In Pinus, at the final free nuclear division, walls perpendicular 

 to the megaspore membrane are formed on the achromatic fibers 

 which connect the nuclei. A wall is also formed on the side next the 

 megaspore membrane; but the sixth side, toward the center of the 

 megaspore cavity, remains open.''^' 34s As division continues, peri- 

 chnal walls come in, but the innermost sides of the innermost cells 

 remain open. This method continues, with the tissue advancing 

 toward the center, even after archegonial initials have appeared at 

 the top. When the central cavity is entirely closed, walls are formed 

 on the centripetal ends of the cells. Consequently, along the line of 



