20 DOUGLAS HOUGHTON CAMPBELL ON THE 



frequently occurs in the male prothallia, a neAV apical cell, dividing in the same way as 

 the original one, maybe formed at the end of a branch. One case was observed Avhere 

 the prothallium was divided into two bi-anches, each having a perfect apical cell, the 

 whole looking very much as if these two cells had originated from a bipartition of the 

 original apical cell. 



Soon after the obliteration of the apical cell, or sometimes before, the cells in its vi- 

 cinity, which hitherto have divided in two planes only, now divide also by walls parallel 

 to the surface of the prothallium, thus giving rise to a mass of cells occupying the bot- 

 tom of the cleft in its front. In cases Avhere the prothallium continues to grow for a 

 long time and this process begins early, a thickened mid-rib is formed, running for 

 nearly its whole length. 



The foregoing account applies only to the larger prothallia, which are, for the most 

 part, female. Many exceptions occur in the male prothallia, and the younger prothallia 

 that are exposed to unfavorable conditions. If the spores are germinated in water, the 

 cells become much longer and more slender, sometimes attaining four or five times the 

 normal length without dividing. On the other hand, prothallia grown under normal 

 conditions are to be met with in which no protonema is formed, but the second cell of 

 the prothallium dividing by a longitudinal wall, the flattened form is assumed almost 

 from the beginning. Where the spores ai'e sown thickly, the male prothallia sometimes 

 do not pass beyond the protonemal stage, the row of cells being terminated by an anthe- 

 ridium, and these simple jjrothallia are sometimes destitute of any i-oot-hair (PI. 4, fig. 

 15). "While many of the male have much the form of the female, differing simply in 

 their smaller size, they ai-e more commonly of very ii'i'egular shapes, especiall}' if they 

 have attained some size, often being much branched and lobed, reminding one strongly 

 of those of certain species of Equisetum. In some irregular forms, either no definite 

 apical cell is formed, or else it is early lost. 



Under favorable circumstances the first antheridia are mature in from five to six 

 weeks from the time the spoi-es are sown, and the first ai-chegonia about three weeks 

 later. "With care they can be kept alive for a comparatively long time. A quantity of 

 prothallia raised from spores sown Aug. 31 , 1884, were kept through the winter, grow- 

 ing but little for about two months, from the last of November. They were removed 

 to a warm room in January and given abimdant light and moisture. Growth was re- 

 sumed and an abundance of antheridia and archegonia wei'e produced. The greater part 

 of the female prothallia produced young plants, but a few that Avere not fertilized were 

 alive a full year from the time that the spores were sown. Most of the male i)rothallia 

 had died a month or two previously. As the growth of the prothallium continues, new 

 root-hairs are formed from the cells near its base; these arise sometimes from the mar- 

 gin, but more commonly from the lower surface of the cells; in exceptional cases from 

 the upper surface. 



Section II. The Sexual Organs. 



The antheridia originate in the same Avay as the root-hairs. In the young prothallia 

 they arise from the marginal cells; in the older ones, from the upper and lower surfaces 

 as well. They begin as small prominences, usually close to the wall separating two ad- 



