142 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



place, which is indicated by shortening of the shoot-axis and other signs, 

 and that finally death ensues ; the relationships are here complicated 

 by the concatenation of numerous generations of shoots. In other cases 

 however internal causes are certainly operative in limiting the development, 

 and of these the relationships of correlation between the generative and 

 the vegetative cells are probably the most effective. 



Of this nature is the case of the prothallus of Selaginella. The macro- 

 spore can take only water from the outside ; the development of the 

 prothallus is therefore limited because it does not produce chlorophyll. 

 The food-material it contains is utilized for the formation of a number of 

 archegonia which arise apparently independently of light as the material 

 necessary for their construction was derived from the sporiferous plant. 

 But the growth of the prothallus of Salvinia is also limited, and yet it 

 possesses chlorophyll and continues to grow if the first-formed archegonia 

 are not fertilized. The material formed by its assimilation is probably 

 however always devoted to the formation of archegonia and it therefore 

 cannot exhibit a strong vegetative growth ; the prothallus therefore ulti- 

 mately dies. The prothalli in the Marsiliaceae also show a continued 

 vegetative growth if the archegonia remain unfertilized although this is 

 limited in point of time. The energy which exists in the prothallus is 

 exhausted by the formation of the archegonium which is its chief duty, 

 and new energy cannot be again added to it. In the higher plants also 

 in certain circumstances the material contained in the seeds may be 

 utilized for the production of flowers and fruits ; at least seedling plants 

 placed in sterile soil with access to light may form a few seeds, but this 

 is only the case in plants which have a rich store of reserve-material. 

 Analogous cases are known also in lower plants. Under unfavourable 

 conditions the development of a germ-cell may be entirely limited to the 

 production of another germ-cell ; for example, the spores shed from 

 Empusa Muscae if they do not reach a fly produce a short germ-tube 

 almost the whole protoplasmic content of which is devoted to the forma- 

 tion of a new spore. The spores of Cladosporium form a sporophore, 

 or even directly new spores, instead of a mycelium, if they be cultivated 

 in conditions which exclude as completely as possible all nutritive material 

 but allow access of light and sufficient moisture 1 . The spores of Mucor 

 racemosus if they germinate in distilled water form a feeble mycelium upon 

 which a small sporophore may arise, and Klebs observed 2 , in experiments 

 carried on in rarefied air, that an individual spore lying in the sporangium 

 was developed into a small but normal sporophore. The cells of the 

 gemmae of Lejeunia Metzgeriopsis, which normally grow out into a 



1 Schostakowitscb, in Flora, Ixxxi (1895), p. 370. 



2 Klebs, Die Btdingungen der Fortpflanzung bei einigen Algen und Piken, p. 496. 



