1524 A TEXTBOOK OF THEORETICAL BOTANY 



capacity of species. For the world as a whole there is all too little information 

 available on these points, vital though they are to any proper appreciation 

 of the life and survival of plant species. For the British Flora, Salisbury 

 has made a most valuable contribution in his book entitled " The Repro- 

 ductive Capacity of Plants". Much of his information is too detailed for 

 citation but some of his general conclusions may be quoted. If the variation 

 of weight among seeds of a species be plotted for several different types of 

 habitat, it is found that the mode of the curve always corresponds to a 

 higher value the more shaded the conditions with which the developing 

 seedling has to contend. The main exceptions to this rule are parasites and 

 saprophytes and plants of mycorrhizal habit. The conclusion drawn from 

 these observations is that the capacity to colonize in the face of competition, 

 that is into advanced communities, is associated with increased amounts 

 of food reserve in the seeds. As polyploids tend to have heavier seeds than 

 diploids they may be specially important in ecologically advanced plant 

 communities. 



Seeds derived from parent plants of very different degrees of vigour 

 show no significant difference in viability or in the vigour of the seedlings. 

 The main effect of poor conditions is to reduce the amount of seed formed 

 but not to affect its quality. It would appear that among ecologically com- 

 parable species, the specific reproductive capacity is an important deter- 

 mining factor with regard to frequency and abundance. On the other 

 hand, the reproductive capacity has little relation to the respective risks of 

 mortality and it appears that the seed output is usually high enough to be 

 well above the minimum safety limit. 



The lowest seed outputs are characteristic of the herbaceous shade 

 plants of woodland undergrowth, the difficulties of the conditions experi- 

 enced being met by increased seed weight and the development of vege- 

 tative propagation. The highest seed outputs characterize those plants, 

 the " opportunists", which grow in habitats only intermittently available, 

 such as clearings in woods. The biological success of a species appears to 

 be directly related to its reproductive capacity. 



FRUITS 



The definition of a fruit presents certain difficulties. It is customary 

 to say that it is the product of the ripening of the gynoecium of the flower, 

 not, be it noted, of the ovary only, for styles and stigmas may both play a 

 part in fruit formation. This is the strict, or as one may say, the " botanical " 

 idea of a fruit, but there are many instances in which other organs con- 

 tribute to the formation of the fruit, such as the calyx, the floral receptacle, 

 the flower pedicel or even the inflorescence axis. These or other organs 

 may develop simultaneously with the development of the gynoecium and 

 become so much an integral part of the ripened fruit that it is practically 

 impossible to separate their contribution to the structure from that of the 

 gynoecium itself. The usual botanical custom is to distinguish these 



