86 



RELATIONSHIPS OF SYMMETRY 



an involution of the vegetative point in the alga Cliftonaea pectinata, and 

 Polyzonia jungermannioides shows the involution to an even greater 

 degree ; amongst the Bryophyta, examples are found in Mastigobryum and 

 Hypnum crista-castrensis ; amongst the Pteridophyta in Azolla ; amongst 

 the Spermaphyta, in the aquatic species of Utricularia. 



The dorsiventrality of vegetative 

 shoots, and especially the position of 

 their organs, can usually be brought 

 into evident relationship with the con- 



FIG. 40. Cliftonaea pectinata. Apex of one of the dors i- 

 ventral shoots. 



FIG. 41. / Riella Battandteri. Upper part of a 

 plantlet. At <5 the position of an emptied anther- 

 idium. At a young sporogonium. v vegetative 

 point. Magnified. // Scheme of Riella. ///Scheme 

 of another thallose liverwort ; the thallus seen in 

 transverse section. 



ditions of life, and the examples we are about to quote will show this 

 without further explanation. Such a relationship is not however always 

 visible, probably on account of our incomplete knowledge of the conditions 

 of life in the plants concerned, although it is at the same time possible 

 to conceive that dorsiventral configuration depends upon ' inner ' causes 



