154 BOTANY 



situated to the right and left of each vascular strand, the rows are 

 arranged in pairs with wider intervals between each pair. 



Inasmuch as a multicellular plant begins its development as a 

 single cell, either from a spore or fertilised egg, and then gradually 

 passes into its multicellular condition with corresponding internal and 

 external differentiation, it repeats in its ontogeny the steps of its 

 PHYLOGENETIC development. These phylogenetic processes, however, 

 undergo material modification in the course of the ontogenetic develop- 

 ment of a plant. The internal modifications are in some respects less 

 marked than those experienced by the external organs, because the 

 internal inherited structure is less subject to the disturbing action of 

 external influences. The ontogeny of the internal differentiation of 

 a plant is on this account often of service in determining its relation- 

 ships. In most cases, it may be safely said that every change in the 

 internal differentiation of an organ is of more general significance the 

 earlier it manifests itself in the development of the embryo, and the 

 nearer it occurs to the growing point in which the embryonic de- 

 velopment is continued. Conversely, a characteristic is so much the 

 more significant for the determination of immediate relationships, 

 the later it makes its appearance in the ontogenetic development. 



Structural Deviations 



Plants, even of the same species, never exactly resemble each 

 other. Every individual organism has its own peculiar character- 

 istics by which it may easily be distinguished from every other of the 

 same species. To a certain extent individual variability may be due 

 to atavism, or the reappearance of previous ancestral qualities. The 

 greater part, however, of such individual variations are the result 

 of newly developed peculiarities. Variations which are inherited lead 

 to the development of new varieties. Independent or spontaneous 

 deviations are often the cause of monstrosities, and as these are apt to 

 disturb the regular functions of an organ they are frequently the occasion 

 of disease. The study of the abnormal development of plants is called 

 Phytoteratology. That a plant becomes abnormally developed may 

 be due either to internal or to external causes. As an example of 

 variations occasioned by internal causes may be cited the so- 

 called bud-variations, which result in the abnormal development of 

 single shoots. In like manner a variation in the number of the 

 members of a floral or leaf whorl may occur as a result of internal 

 causes ; thus, for example, Paris quadrifolia occasionally exhibits a 

 hexamerous instead of a tetramerous symmetry. The internal struc- 

 ture of a plant may likewise be disturbed, and the development 

 of its vascular water-courses or of its mechanical elements become 

 considerably altered. In many cases variations are, no doubt, the 

 result of changes in the mode of nutrition ; this fact has been 



