Sm J. LUBBOCK — PHTTOBIOLOGICAL obseryatioxs. 391 



things in Avhich every seed grew and attained maturity, then to 

 keep up the number of any given species existing at any time, it 

 would be sufficient if each 2)lant produced but one or two seeds 

 during its whole life. There is, however, an enormous destruction 

 of seeds. The great majority are eaten by animals, or fail to secure 

 a suitable site for germination ; of those which do germinate, again, 

 many are crowded out bv their fellows; and Darwin observed that 

 out of 357 seedlings which came up in a space of 3 feet by 2, 

 no less than 295 w ere destroyed by slugs and insects. The greater 

 the chance against any given seed reaching a suitable locality and 

 attaining maturity, the larger number of seeds must the plant 

 produce in order to maintain its numbers^ and, as a general rule, 

 the smaller will the individual seeds be. On the contrary, the 

 greater the chance that each seed enjoys of arriving at maturity, 

 the smaller the number of seeds that is necessary, and in such 

 cases it is an advantage that the seeds sliould be large. 



Hence parasitic plants generally produce a large number of 

 very small seeds, though there are exceptions due to other consi- 

 derations, as, for instance, in the Mistletoe (I believe, indeed, all 

 theLoranthacese), where the seeds are carried by birds. 



An interesting illustration is afforded by certain species which 

 produce two kinds of pods, as, for instance, Cardamine cJieno- 

 podifolia of Brazil. Besides ordinary pods, which resemble those 

 of any other Cardamine, and contain several seeds, this plant pro- 

 duces a second sort of pod underground. Is^ovv in the ordinary 

 pods the number of seeds increases, of course, the chance that some 

 one will i3nd a suitable 2)lace. On the other hand, the subterranean 

 pods are sown, as it were, by the plant itself. In this case, if 

 there were a number of seeds they would only get in one another's 

 way, and hence, perhaps, the fact that the subterranean pods 

 only produce one or two seeds. In most species the seeds vary 

 somewhat in size ; hut in such cases it would not be conclusive 

 to contrast the produce of large seeds with that of smaller ones. 



because it might fairly be saiJ that the former were better 

 nourished, and inherently, perhaps, more vigorous. In £^arda- 

 mine clienopodifulia of Brazil, however, the seeds from the under- 

 ground pods are larger than the others, and Grisebach found that 

 they produced more vigorous seedlings *. 



There are, on the contrary, other considerations which may 



* "Der Dimorphismus der Fortpfl. v. Cardamine cTicnopodiJolial' Gottinger 

 Nachricbtungen, 1878, p. 334. 



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