472 



HETEROMORPHISM AND ALTERNATION OF GENERATIONS. 



fruiting year is followed, in these trees, by several sparing ones; so much so that 

 it suggests that the trees are exhausted by the heavy production and require 

 time in which to recover, and, by the formation of non-flowering shoots with 

 green foliage, to manufacture and lay by' stores of food-material. So also in 



Fig. 346.— Alternation of Generations in Ferns. 



1 A Fern-prothalliura seen from the under side; archegonia are present amongst the rhizoids and towards the sinus at the top, 

 antheridia on the margin below. - Longitudinal section of an archegonium showing the egg-cell (shaded) in its ventral 

 portion. The canal leading to the egg occupies the neck-portion. 3 Longitudinal section of an antheridium showing the 

 spermatozoids coiled up within. * Antheridium discharging its spermatozoids. 5 Commencement of the asexual genera- 

 tion. The first simple frond of the young fern-plant (sporophyte) is held aloft, whilst a root descends into the ground. 

 The young fern-plant is still attached to the prothallium. 6 Complete sporophyte of the Wall-rue Spleeuwort (Asplenium 

 Ruta-muraria) with its fronds showing sori. ' Under surface of a pinnule of the sporophyte of the Wall-rue Spleenwort 

 (^Asplenium Ruta-muraria) showing the linear aggregations of sporangia (sori), with lateral indusia. * A young prothallium 

 arising from a spore ; the spore is below, s natural size; i x 8 ; 2, 3, and * x 350 ;*x6:'x3;8x 240. 



many low herbs. Now and then the Orchids in the meadows flower in immense 

 profusion, and we say it is a good " Orchid year " ; then follow years in which, 

 in the same localities, hardly an orchid-flower is to be found. 



