THE STEM OR SHOOT. 



145 



clearly, in one sense, transitional structures between a 

 fern-leaf and a prothallus-lobe, as are also the tips of 

 the pinnules which merge into prothalli; they are not, 

 however, in the morphological sense of the term 

 transitional organs at all ; there is no series of forms 

 connecting the two, as e. g. there is between a phyllode 

 and a compound leaf; a transition only exists be- 

 tween one portion of the prothallus-lobe and another 

 portion of the same lobe. If there could be shown an 

 actual bud on the prothallus bearing both prothallus- 

 lobes and leaves or structures combining tlie characters 

 of these two organs, then we might believe in the 

 possibility of a transition between a prothallus and a 

 sporophyte shoot. The real explanation of the ab- 

 normal structures so happily discovered by Lang 

 appears to be as follows. If we consider, for the sake 

 of comparison, the formation of an adventitious shoot 

 upon, say, a Begonia-leaf, it is obvious that, in order 

 to form the shoot at all, transitional stages of growth 

 must be gone through representing the passage 

 between the tissues of the leaf at that particular spot 

 and those of the shoot ; these are passed through so 

 rapidly, however, as to be unnoticeable. The same is 

 true of many of the buds formed on the prothallus 

 and of the prothallus formed on pinnules. But, for 

 all that, these intermediate stages, representing the 

 adaptation of the tissues of the leaf or prothallus 

 respectively to the formation of those of the shoot, 

 must exist, however unnoticed. On the other hand, 

 the " new growths " on the prothallus appear to owe 

 their existence to the fact that in many cases the 

 transitional stages between the tissue-formation of 

 the prothallus and that of the shoot are not sudden, 

 but, on the contrary, long drawn-out, and often, more- 

 over, interrupted ; the attempt on the part of the 

 prothallus-tissue to form sporophytic shoot-tissues is 

 also often half-hearted and incomplete, often result- 

 ing in failure, when . a reversion to the formation of 

 prothallus-tissue once more supervenes. Phenomena 

 vol. i. 10 



