i8 GENERAL DIFFERENTIATION OF THE PLANT-BODY 



We might say very much the same about other organs of propaga- 

 tion, such, for example, as the nucellus of the ovule, and the antheridia 

 and the archegonia of the Archegoniatae. The antheridia of mosses 

 are formed, as is well known, sometimes as terminations of the 

 apex of the stem, sometimes in the position of leaves or hairs. What 

 this teaches us is that the place of origin varies, and if, looking at the 

 fact, we say that the ' morphological significance ' of the antheridium 

 varies, we can only mean that the morphological significance is in this 

 sense of subordinate importance. The organs of propagation naturally 

 arise on preceding vegetative organs, and the vegetative organs which 

 produce the propagative organs are frequently transformed in a character- 

 istic manner. But the propagative organs cannot be referred back to 

 vegetative organs. Looked at from the phylogenetic point of view they 

 have been in existence, although in a simpler form, before the vegetative 

 body reached the differentiation it now possesses. Amongst the Sperma- 

 phyta in particular the structures bearing the propagative organs have 

 not been sufficiently distinguished from the vegetative organs. A stamen, 

 for example, is considered an individual structure, and yet it consists of 

 a sporophyll and a sporangium (pollen-sac), which is often sunk in it. 

 The sporophyll is a transformed leaf 1 ; and with regard to the sporangium, 

 while it is absurd to view it as a transformed part of a leaf, it is a weighty 

 accession to our knowledge to determine it to be the homologu of the 

 sporangium in the Pteridophyta. The expression ' homologue ' may be here 

 further explained, for it is used in different senses which can be well 

 illustrated by consideration of the stamens. When I say a stamen 

 is the homologue of a leaf, the pollen-sac is the homologue of a spor- 

 angium, or, if you will, of a row of sporangia the term has not the 

 same signification in the two cases. A pollen-sac is a sporangium because 

 of its function inasmuch as it produces spores, and it occupies in the 

 whole plant-economy of one of the Spermaphyta the position which 

 a microsporangium holds in that of a Selaginella. A stamen is a trans- 

 formed leaf which has acquired another form and function because it 

 produces sporangia. Had all the Pteridophyta, especially the hetero- 

 sporous ones, been destroyed, we should not have been able to determine 

 the correspondence of the pollen-sac with the sporangium ; on the other 

 hand, were the earth covered with Spermaphyta alone, we should be able 

 to ascertain that a stamen is a transformed leaf. It is necessary to dis- 

 tinguish these two points of view. Homologous organs are commonly 

 defined as those for which we can trace a common phylogenetic origin, 

 far back although this sometimes goes. But careful investigation cannot 

 fail to convince us that many different lines of development have 



1 For a discussion of opposite views, especially that of Bower, see Part II of this book. 



