1038 REPORT— 1898. 



and apospory has not been directly followed. But if the nuclear diiference 

 between the two generations be as it is believed, nuclear changes will be closely 

 connected with these vegetative transitions. What could appear more natural 

 than that apogamy, which presumably involves a doubling of the chromosomes, 

 should follow a condition of plethora, and that apospory, which presumably 

 involves a halving of the chromosomes, should follow deficient nutrition ? 



One further fact in either case appears to me to be specially noteworthy, that 

 the changes are not confined to a single cell. The directly apogamous bud of 

 Nephrodium Filix mas may perhaps be referable to a single cell, but Dr. Lang 

 shows by numerous examples that the transition from characteristic tissue of the 

 gametophyte to that of the sporophyte may arise at various points, and involve 

 considerable tracts of tissue. Similarly, I have shown in the case of apospory 

 that the change may affect not one cell only, but cell-groups at various and dis- 

 tinct points on the same individual. It would seem that there is a widespread 

 disposition of the tissues to undergo the change. 



For my own part, I think the usual attitude on the chromosome question has 

 been too absolute and arithmetical. Evidence is accumulating from various sources 

 that the usual numbers are not strictly maintained ; it is known that in vegetative 

 cells there are often considerable differences of the number of chromosomes 

 from those in the sexual cells of the same plant, while observers have noted 

 the irregularities in the divisions of the pollen-mother-cells in such plants as 

 Hemerocalli» and Tradescantia. If there be any causal connection between the 

 number of chromosomes and the morphological character of the sporophyte and 

 gametophyte, irregularities such as these at least countenance the idea of nuclear 

 instability being possible ; it will be a question for special treatment and investi- 

 gation how far nuclear instability is connected with disturbed nutrition. But 

 into the mechanism of the presumable nuclear change, and the question whether 

 it be sudden or gradual, we cannot enter with any more than a speculative 

 interest, in the absence of direct observations. Whatever the nuclear details may 

 be, I regard it as a matter of very great importance to recognise that special condi- 

 tions of nutrition commonly accompany, if indeed they do not actually determine, 

 those changes which we term apospory and apogamy. But the story of the past 

 is not simply a matter of conditions of nutrition, as we see them now influencing 

 Archegoniate plants in their present highly specialised state. The real question is 

 a purely historical one, How did the present state of things come about ? 



The following considerations influence me in forming an opinion as to the real 

 place of apospory and apogamy in the history of the alternating generations : — 



I. The Bryophytes show remarkable imiformity of alternation : irregularities 

 are few ; apogamy is not recorded ; apospory appears rarely, as a physiological 

 refuge for the destitute plant. This uniformity goes along with the protected and 

 dependent condition of the sporophyte. All Pteridophytes have their embryos 

 protected while young, and this seems to have been their primitive condition. 

 The true lesson of the Bryophyta, which include the simplest living Archegoniates, 

 seems thus to be that uniformity of alternation goes with a simple structure, and 

 a protected or dependent condition of the sporophyte ; and this we have reason to 

 believe was the condition of the simpler Archegoniate fruits. 



II. The distribution of apogamy and apospory among Archegoniates at large is 

 very irregular ; the Leptosporangiate Ferns are the head-quarters ; but they are a 

 peculiarly specialised phylum, with free sporophyte, exposed when mature, though 

 protected while young. They are adapted to special conditions and show a 

 greater plasticity of development than any other Pteridophytes. The Ferns are 

 subject to other abnormalities than apospory and apogamy. The root may de- 

 velop directly into a shoot, or the apex of the leaf into a bud. I think it has been 

 too readily held that the Ferns occupy a special place as a key to the morpho- 

 logical problem. W^e should bear in mind how really isolated they are ; they are 

 essentially an extreme, even an extravagant type ; they show the largest sporo- 

 phylls in the whole vegetable kingdom, with the largest numerical output of 

 spores from each. Many are specialised in accordance with extreme conditions of 

 shade and moisture. These considerations should temper our view of them, not 

 only as material for normal comparison, but also as exponents of abnormality. 



