DEVELOPMENT OF THE OSTRICH FERN. 39 



which forms the medullary parenchyma of the stem, is joined to the periblem from which 

 the cortical parenchyma and hypoderm arise. The proeambium has thus the form of a 

 hollow c} Under, with openings at regular intervals, and, as the proeambium develops 

 into the elements of the complete bundle, the whole assumes the form of a C3dindrieul 

 net, apparentl}' formed by the union of separate bundles, but being stricth* a single 

 bundle. 



Shortly after the proeambium begins to form, the central part of the plerome begins 

 to show the formation of the cavities that characterize the central parts of the stem. At 

 certain points the cells become longer and their contents less dense, so that the places 

 appear more transparent than the surrounding tissue. The cells cease to divide, or at 

 least do so more slowly than the surrounding ones. By the rapid growth of the sur- 

 rounding tissue and the consequent widening of the stem, certain of these cells are torn 

 apart, leaving intercellular sj)aces that become rapidly larger as the stem widens. These 

 spaces are bounded by loosely ari'anged cells whose free ends are rounded and with 

 nearly colorless contents containing a few definite granules and a distinct nucleus. 

 From these cells trichomes of different forms are develojied. These may be in the form 

 of simple hairs or scales of considerable size. Sometimes, but not very often, they may be 

 tipped with a jjear-shaped glandular cell, tilled with dense granular mucilage. 



The mass of trichomes filling up the cavity now dries up, and the cells bordering them 

 have their walls somewhat thickened and changed to a brown color. Thus, there are 

 formed large air spaces distributed through the stem. Very often these communicate 

 with the spaces between the bases of the leaf-stalks and the stem, hy the destruction 

 of the intermediate tissue, brought about in the same way as the formation of the cavi- 

 ties themselves, the cavities thus being brought into communication with the external 

 air. The communication between the inner and outer cavities takes place through the 

 foliar gaps. Finally, the cells about the cavity acquire thicker walls, which are at first 

 yellow in color, but finally dark bro^\ai, forming an imperfect scIerenclBana. 



The remainder of the medulla develops into a mass of parenchyma, densely filled with 

 starch. The walls gradually become thicker and develop simple pits. 



The periblem, which has rather longer cells, separates into two portions, but not until 

 a later period. Its cells become longer and finally several of the outer layer become 

 still longer, their walls also becoming thicker and the contents less opaque. These con- 

 stitiite the hypoderm, and finally become very sharjily separated from the underlying- 

 cortical parenchyma. Their lateral walls are somewhat uneven, and all the walls swell 

 up more strongly on tiie application of potash than those of the parenchyma. The walls 

 of the hypodermal cells next assume a yellow color, which becomes later much darker 

 and the wall finally develops pits which extend to the intercelhdar lamella which is very 

 clearly defined and appears darker than the rest of the wall. This sclerenchyma, 

 although well-developed, does not attain as perfect a form as is found in some other 

 ferns. 



The epidermis of the stem never reaches any degree of perfection, as only a very small 

 part of the surface of the stem is uncovered, and none of it in direct contact with the 

 air. Whatever epidernns there is soon dries up and l)ecomcs brown like that of the 

 root. 



