446 THE BIOLOGY OF FLOWERING PLANTS 



at the transition stage has a highly peculiar appearance 

 (Fig. 68). The youth form very rarely flowers. 



The youth form tends to be more mesophytic than the 

 adult, which, in the cases cited, is a pronounced xerophjrte. 

 This may be related to the diff'erent conditions in which the 

 seedling grows, in the shade of other vegetation, or at a 

 damp season of the year. In many cases, too, the youth 

 form may be looked on as ancestral in type ; in the whin and 

 Acacia, bearing youth leaves of a form characteristic of most 

 members of the family, this is certainly so. Of the Retini- 

 sporas Goebel says : " As the youth forms here, as in Pinus, 



are without doubt to be 

 regarded as primitive, we 

 have thus been able in a 

 sense to bring to life again 

 the ancestral type." 



This is not always the 

 case. Goebel regards the 

 adult form of the ivy as 

 ancestral. Growing on a 

 wall, or in the shade of a 

 wood, the leaves, borne in 

 two rows, are deeply lobed 

 and dark green in colour. 

 When the climber grows 

 on to the roof, and receives 

 more uniform and stronger 

 illumination, erect shoots are formed bearing large, pale, 

 sUghtly lobed, or entire leaves in a one-third or two-fifths 

 spiral ; only on these shoots are flowers produced. Especially 

 in the phyllotaxy the adult shoot resembles the type normal 

 in other members of the Araliaceae. 



Another group of plants with well-marked youth forms 

 are the aquatics. The pond- weeds and water-lilies always 

 produce first a number of linear, or band-shaped, submerged 

 leaves. In Alisma plantago and Sagittaria sagittifolia the 

 narrow submerged leaves are followed by a few floating 

 leaves, and then by the free oval, or arrow-shaped subaerial 



Fig. 68. — Thuja, showing transition 

 from the youth leaf -form. X i§. 



