144 DIFFERENCE OF ORGANS AT DEVELOPMENT STAGES 



homoblastic development (Fig. 84). The shoot-axes of this plant contain 

 chlorophyll and, like those of Equisetum, have rudimentary cyclic leaves 

 concrescent in a sheath. This character appears in the seedling above 

 the cotyledons. The whorl following the cotyledons is two-membered 

 and the leaves are at right angles to the cotyledons ; thereupon follows 

 a second decussating two-membered whorl ; the third is four-membered, 

 and the members cross those of the second diagonally l . The horse-tail 

 itself presents the same features : The axis of the germ-plant is much 

 slenderer and has a simpler anatomy than the axis of the later shoots ; the 

 number of leaves in a whorl is smaller ; subterranean shoots are wanting ; 

 and it forms shoot-generations which are successively stronger 

 until the definite adult form is reached, but the outline of 

 the configuration is the same in all shoots. 



Heteroblastic development is exhibited by the Australian 

 species of Acacia which possess phyllodes, and is also seen elsc- 

 where especially in Bryophyta and many Algae. Examples 

 of this have specially attracted attention when, as happens 

 in the case of Acacia, the configuration of the seedling 

 resembles that of the adult form in allied plants. Many 

 species of Acacia have permanently the leaf-form which 

 appears only at germination in those producing phyllodes, 

 and the configuration of the seedling places directly before 

 our eyes the transformation which has taken place. The 

 retention by the seedling in this case of the original, phylo- 

 genetically older, form of the vegetative organs is connected 

 with 2 its living under other conditions than does the adult 

 form. But in other cases the configuration of the seedling 

 certainly does not turn upon the retention of an original 

 relationship of form, but is an adaptation 3 developed later, and 

 the seedling exhibits then derived, not primitive, characters. 

 FIG. 84 Seed- The adaptation to other relationships does not of itself 



ling plant of Cas- 



uarina tomiosa. furnish us with any explanation of the character; this is 



Magnified 2. J 



obtainable only by comparison with allied forms. In many 

 cases too the distinction we have just pointed out cannot be drawn 

 with any certainty ; within one and the same cycle of affinity, and 

 even within one and the same genus, the configuration of the juvenile 

 form is not always similar. 



1 See Morini, Contribute all' anatomia del caule e della foglia delle Casuarinee, in Mem. della R. 

 Accad. delle scienze dell' istituto di Bologna, ser. v. T. iv. p. 692. 



2 See below on p. 153, and following pages. 



3 Such are the different contrivances for nutrition of the tmbryo and for facilitating germination, 

 about which, so far as they have morphological interest, some account will be ghen in Part II of 

 this book. 



