MORPHOLOGY 165 



must be altered ; the organism with the single number of chromosomes may be 

 termed the haploid, or haploid generation, that with the double number the 

 diploid, or diploid generation. 



The union of paternal and maternal chromosomes in each sexual act results in 

 the nuclei of the offspring being provided with maternal and paternal chromosomes 

 in equal numbers. All the nuclei of a diploid generation contain each kind of 

 chromosome in duplicate in two equivalent chromosomes. In the reduction division 

 by which the formation of the reproductive cells is prepared for, the chromosomes 

 become united (as hag been seen on p. 84) in pairs during the prophase.of the. 

 division. It is to be assumed that the pairs are formed of the corresponding 

 parental chromosomes. In the separation which follows during the metaphase of 

 the reduction division each chromosome may apparently pass to either of the 

 two daughter nuclei, but always in such a manner that the chromosomes of -each 

 pair pass to different nuclei. Thus on the one hand, all combinations of the 

 parental chromosomes are possible in the products of this division ; on the other 

 hand a chromosome of eacli kind is ensured to each of the resulting nuclei. All 

 these morphological processes, which are matters of observation, lend great support 

 to the assumption that the nuclei are the bearers of the inherited properties of the 

 organism ( 159 ). 



By those apogamous processes which exclude a reduction division (p. 93) an 

 alternation of generations is also abolished. 



Structural Deviations ( 16 ) 



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

 other. Every individual organism has its own peculiar characteristics 

 by which it may be distinguished from every other of the same 

 species. To a certain extent these characters may be due to 

 ATAVISM, or the reappearance of previous ancestral qualities. Most 

 individual deviations belong, however, to the so-called FLUCTUATING 

 VARIATIONS. These occur in all species, and may be compared to the 

 excursions of a pendulum to either side of its position of equilibrium. 

 Abrupt and striking deviations of the individual from the type of 

 the species are called MONSTROSITIES. When these induce a dis- 

 turbance of function a diseased condition results. The causes of 

 monstrosities may lie in the organism itself ; in other cases they can 

 be shown to stand in relation to external causes. Among EXTERNAL 

 CAUSES OF VARIATIONS the influence exercised by parasites upon the 

 development of the portions of the plant is particularly striking. 

 Euphorbia Cyparissias, when attacked by a rust fungus (Aecidium 

 Euphorbiae), becomes sterile, remains unbranched, has shorter and 

 broader leaves, and in its whole appearance is so changed as scarcely 

 to be recognisable. Plant lice sometimes cause a flower to turn 

 green, so that instead of floral leaves green foliage-like leaves appear. 

 Another peculiar example of abnormal growths is afforded by the 

 GALLS or CECIDIA produced on plants by Fungi, or more frequently by 

 insects, worms, and arthropods. The effect of these formations on the 



