QUESTION OF INNER CAUSES 101 



The general features of the change from a sporogenous to a sterile 

 character are associated usually with a less dense protoplasm and a 

 smaller and less marked nucleus. If disorganisation be the ultimate fate, 

 the wall breaks down, and the identity of the protoplast is lost, with or 

 without fragmentation of the nucleus, as in Psilotum : or the proto- 

 plasm may shrink and collapse, and the whole protoplast become 

 highly refractive before the final absorption, as in angiospermic ovules. 

 If the cell is to continue functional in a vegetative capacity, the changes 

 are those usual in cells passing from the embryonic to the mature condition. 

 It has been stated above that the occurrence or absence of the tetrad- 

 division, and of the consequent chromosome-reduction, is the ultimate 

 criterion of distinction between a fertile and a sterile cell : in the majority 

 of cases the distinction has been drawn on the basis of the results of 

 subsequent development, not on that of actual observation of the nuclear 

 changes. It is not, however, probable that this has led to any serious 

 errors, since the tetrad-formation which follows on chromosome-reduction 

 is a sufficiently distinctive feature in all cases except in the ovules of 

 Seed-Plants. This being so, it is not surprising that the most exact nuclear 

 observations of sporogenous cells, in which the sterile or fertile development 

 is a critical question, have been made on the ovules of certain Angiosperms, 

 viz. in the case of the apogamous species of the genus Alchemilla. 1 The 

 exact questions connected with these plants do not come before us here ; 

 but in their elucidation Strasburger had reason to follow carefully through 

 the development of certain embryo-sacs, as regards their nuclear condition. 

 He found that an archesporial cell having entered the condition of an 

 embryo-sac-mother-cell, its nucleus passes through the prophases of the reduc- 

 tion-division, up to the stage of synapsis. The embryo-sac-mother-cell then 

 alters its trend of development and becomes vegetative, and its nucleus 

 passes out of the synapsis condition into that of a typical division, instead 

 of continuing the reduction-division. The cell thus remains a part of the 

 tissue-system of its parent, not the initial cell of a new generation. Such 

 a case is interesting in that it shows how a cell may tremble on the verge 

 between the sterile and the fertile state. It leaves, however, still open 

 the question as to the influences, external or inner, which determine its 

 fate. These probably vary in different cases, and the problem would 

 naturally be a simpler one in the Homosporous Archegoniatae than in the 

 ovule of an Angiosperm. It seems obvious in the simpler cases to suggest 

 nutrition as one potent factor : it is a necessary axiom that an increasing 

 spore-output, which is an advantage in increasing the probability of survival 

 and dissemination, demands increased nourishment and protection : and 

 that a vegetative system increased by sterilisation will tend to provide this. 

 But still the advantage gained may be quite independent of the real 

 cause : we are not yet in a position to translate the nutritive demand into 

 terms of a direct influence upon the individual cell. It seems useless to 

 1 Strasburger, "Die Apogamie der Eualchimillen," Pringsh. Jahrb., Band xli., Heft i. 



