TRANSACTIONS OF SECTION K. 1061 



Miguel, and others, the view might appear to be suflBciently established, that in 

 the Diatomeae the formation of auxospores follows only as a consequence of that 

 organisation of the cell which has been described after a number of divisions which 

 may be almost mathematically detined, while the external conditions play no 

 definite part in the process. But, meanwhile, we ought not to forget that this 

 theory is in much need of further confirmation. The main point in the whole 

 question is whether the cell-wall of both cells after division really does undergo 

 no increase by growth in length to even a very small degree. Phtzer himself has 

 noted that such growth does occur in certain species, though this would be of no 

 accoimt for making up the loss of size which accompanies division. It ought to 

 be distinctly proved, by direct and exact measurements, whether a growth in 

 length takes place or not; above all, we ought to know precisely what influence 

 the various external conditions exert upon the life of the Diatomese. It may 

 possibly be that under certain circumstances growth takes place, under others not. 

 The fact, brought forward by Miguel and others, that it is by no means always the 

 smallest cells which form auxospores, but also those of middle size, deserves con- 

 sideration. Still more important are the statements made by Karsten, that 

 Melosira nummuloides can be brought to the formation of auxospores simply by 

 change of water, that in Achnanthes longipes the impending conjugation does not 

 take place, but is replaced by vegetative growth when the cells are exposed to a 

 cool temperature. If we assume that definite external conditions induce growth 

 in length during division, others encourage auxospore formation, the earlier obser- 

 vations on the smallness of the cells which form auxospores may thus be explained. 

 In many Thallophy tes the rule holds that in the stage of preparat ion for the higher 

 fruit-form growth diminishes, or ceases, but division is still continued, so that, for 

 instance, in the Desmidiacese, Spiro(/yra, and Chlamydomonas, it is always the 

 smallest cells which conjugate. This may also be the case on the formation of the 

 auxospores of the Diatomese, and the smallness of the cells would then be less the 

 cause of the auxospore formation than the result of those external conditions 

 ■which occasion this process. I bring this possibility forward in order to draw 

 attention to the pressing need for accurately defining the conditions of the events 

 in the life of the Diatomete by the help of pure cultures, and by the use of physio- 

 logical methods. Whichever way the decision may fall, the life-history of the 

 Diatomeee gives no explanation of the wholly dilierent alternation of generations 

 of the Archegoniatae, any more than does that of the other Thallopbytes which 

 have been mentioned. 



But now the question arises whether there is not in many Thallopbytes another 

 form of alternation of generations, which presents nearer relations to the phe- 

 nomena in the Archegoniatae. The fertilised ovum develops according to the 

 statements of investigators in a definite way in certain species : thus, for instance, 

 the zygospore of one of the Mucorinere, the oospore of Vaucheria, and Saprolegnia 

 usually germinates by formation of a short tube, which directly bears a sporangium. 

 Pringsheim speaks, in such a case, of the first neutral generation : we might 

 regard this as the actual spore-forming generation, corresponding to the sporophyte 

 of the Mosses. Closer investigation shows that an oospore of Vaucheria shows no 

 tendency in any way fixed by heredity to form a sporangium. It produces first a 

 short germinal tube, which may either continue its vegetative growth, or may at 

 once form zoospores, or sometimes sexual organs. That would depend alone on 

 external conditions. Be Bary and I myself have lately proved the same for the 

 oospores of Saprolegnia, and Van Tieghem for the zygospores of the Mucorini. 

 There is no true meaning in speaking of an alternation of generations in these 

 cases, since the formation of the sporangia is not a peculiarity of germination, but 

 follows the same conditions <as it does subsequently. These plants do not behave 

 differently in principle from the Fucacese, or Conjugate, in which the fertilised 

 ovum passes more or less directly into the thallus, since no other propagation exists 

 at all. 



But there are perhaps other species in which the mode of germination of the 

 oospores has become more definite. The zygotes of Hydrodictyon show, according 

 to Pringsheim, a characteristic mode of germination, but it is not yet knowa 



