294 METHODS IN PLANT HISTOLOGY 



During the second year most of the metaxylem becomes lignified; and 

 in the third year, hgnification is completed. The first-year and begin- 

 ning second-year stems cut easily in paraffin and are splendid for 

 showing protoxylem, metaxylem, and phloem. The third-year stem 

 can be cut, but not so easily. Dr. Turner's paper should be read by 

 those who wish to understand this interesting stem. 



Stems more difficult to cut, like those of L. ohscurum, L. clavatum, 

 and L. coiwplanatiim, after fixing and washing, had better be treated 

 for 2 or 3 days with 10 per cent hydrofluoric acid. Sections of stem and 

 root of L. complanatum, mounted on the same slide, show an interest- 

 ing parallelism of structures. Transverse sections of the stem of L. 

 pithyoides, a Mexican species, show not only the stem structures but 

 excellent transverse sections of roots which grow down through the 

 cortex. 



If young sporelings are available they afford a beautiful example of 

 a very primitive type of stele, in transverse section showing an exarch 

 protostele with 4 or 5 radiating arms of metaxylem, each tipped with 

 a comparatively large group of protoxylem cells. In most species, this 

 simple radial stele of the sporeling passes into a compHcated, banded 

 stele in the adult plant. Even in the adult plant the protoxylem and 

 metaxylem are easily distinguished in sections near the growing point 

 of the stem or root. 



The strohilus. — For longitudinal sections, cut a slab from each side 

 of the strobilus to insure fixing and infiltration. If a strobilus, or simi- 

 lar organ, is simply halved, both pieces are hkely to curve. Among 

 north temperature species, Lycopodium inundatum is the most easily 

 cut. A young strobilus 1 cm. in length may show all stages from the 

 archesporium to the spore mother-cell. Iron-haematoxylin is the best 

 stain for differentiating the archesporial cells. The divisions in the 

 spore mother-cell stain intensely, so that care must be taken not to 

 overstain. 



Strobili of Lycopodium dendroideum or L. ohscurum 6 or 7 mm. in 

 length show a beautiful series in the development of the sporangium 

 from the earliest stages up to tetrads. They fix well in the Chicago 

 chromo-acetic-osmic formula. 



The gametophyte.— In most species the gametophyte, or prothal- 

 lium, is subterranean, tuberous, and has no chlorophyll; in other 

 species the prothallium is partly subterranean and partly aerial, the 

 aerial portion being green and bearing the archegonia and antheridia. 



