200 BIOLOGICAL LECTURES. 



a small part of the Hepatics. The stalkless sporogonia are 

 borne on specially developed, erect portions of the thallus, 

 which are found only in this single family. The primitive 

 Bryophyte structure and life history are so masked by the mod- 

 ifications which Marchantia presents that good students dis- 

 tinguish between the fundamental and the accessory features 

 with difficulty. A very capable and generally well-trained 

 student at the Marine Laboratory, while studying Pallavicinia 

 this season (1896), exclaimed, as she comprehended its simple 

 and typical life history, " Why, it is just like a moss." Her 

 previous acquaintance with the Bryophytes had been gained from 

 the study of Marchantia and a moss, and their essential simi- 

 larity had been quite concealed by the secondary modifications 

 of the former. Such Hepatics as those above recommended 

 may be found in wet, boggy ground and about springs, and 

 many times repay the additional effort required in obtaining 

 them. Both of those mentioned produce their sexual organs 

 in summer and bear the young sporogonia in autumn. 



It is impossible to give an adequate idea of the Pteridophytes 

 by means of a single type, but if only one can be used there is 

 no doubt what it should be. 



10. Pteris, or some other common fern, illustrates in its large 

 green prothallus and its vascular, leafy, sporangium-bearing 

 sporophyte, the life history of all the vascular plants, and rep- 

 resents one of the earliest stages in the line of development 

 which culminates only in the highest seed plants. If time per- 

 mits, it is very helpful to examine the fertile spikes of a species 

 of Selaginella, either one of our own or of such as are to be 

 found in almost any greenhouse where ferns are grown and 

 their prothalli are obtained. Here is seen a simple case of 

 heterospory, that condition which has arisen in each of the 

 principal Pteridophyte groups, and which is permanent in all 

 the seed plants. A very brief study of such a form makes the 

 passage to the Spermatophytes and their relations to the 

 Pteridophytes much more intelligible. The two groups of 

 Spermatophytes show so little in common, apart from the 

 investment of the embryo by protective and nourishing tissues 

 to form a seed, that each should be studied in one illustrative 



