446 A TEXTBOOK OF THEORETICAL BOTANY 



such as the compact growth, the close overlapping and inrolling of the leaves, 

 and the presence of specialized water absorbing and storing cells. In sub- 

 merged plants, however, many of these features disappear, and it has been 

 concluded that the normal bog-living plants suffer from occasional drought 

 periods when these devices must operate to save the life of the plant. In 

 Nature Sphagnum is highly calcifuge, i.e., it grows only on lime-free soils 

 and is very sensitive to the presence of Calcium salts in culture, though 

 there is a good deal of difference between species in this respect. The plants 

 give an acid reaction to litmus and contain quantities of insoluble organic 

 acids of colloidal nature, like those of peat, which have the power of decom- 

 posing salts, even Sodium chloride, retaining the basic ions and liberating the 

 salt-acids. This appears to be the plant's method of absorbing its mineral 

 nutriment, and it is significant that those species living in water with the 

 smallest amounts of dissolved mineral salts have the greatest content of 

 acid. The effect of Calcium carbonate in solution is harmful only in the same 

 w-ay that other alkalies are harmful, namely, that it produces an alkaline 

 environment which prevents the characteristic acid adsorption reaction and 

 so kills the plant. 



Origin of the Bryophyta 



If we compare the life history of the Bryophyta with the algal examples of 

 the Thallophyta which we have considered, we shall see certain obvious 

 differences. The structure of the thallus, even in the simpler members 

 like Pellia, shows a considerable advance above the Algae, for it is composed 

 of a parenchymatous tissue derived from a single apical cell and not from 

 the association of filaments, each of which grows more or less independently. 

 We see, too, that there is some differentiation of the cells which compose the 

 thallus. In Pellia this differentiation is slight, but as we pass to the more 

 complex forms such as Mnium and Polytrichum we find this differentiation 

 becoming more and more marked. Certain cells elongate and serve to 

 conduct water, and in the most complex members we have seen that a system 

 of conducting tissues may be formed, resembling that in higher plants, while 

 branches of this conducting system pass out to the leaves, the main tract 

 terminating, in the female plant, at the base of the developing capsule. 



When we turn to consider the reproductive organs we find great changes 

 in their morphology, and it is difficult at first sight to see any relation between 

 the antheridium and the archegonium of the Bryophyta and the antheridium 

 and the oogonium of the Algae. The most important change which has 

 taken place has been the retention of the female gamete within the parent 

 plant and its consequent loss of motility. It is true that in the Rhodophyceae 

 the carpogonium is not liberated, and the oosphere is fertilized in the 

 oogonial cell in Vaucheria and in Saprolegnia. But in none of these cases 

 is there any cellular structure surrounding the oosphere comparable to the 

 archegonium (Fig. 437). 



One of the problems which has beset evolutionists has been to explain 

 how the sex organs of the Bryophyta could have been evolved from those 



