8 ANNALS NEW YORK ACADEMY OF SCIENCES 



Normal Stages in Sporogenesis 



Stages of s[i« Ho^ciicsis that are normal in some one particular are 

 frequently to be observed. In PI. II, Fig. 19 there has been disjunction 

 of all the <» pairs of chromosomes but 1 chromosome is lagging. In PI. 

 III. Fig. 32 the shape of the chromosomes is normal and they are in 

 four groups but the distribution has not been equal. In PL III, Fig. 34 

 are shown four sister nuclei of nearly equal size, and each nucleus evi- 

 dently contains 6 chromosomes that are now greatly increased in size 

 and especially in length, but at least some of the chromosomes may be 

 abnormal, for there are present some of the enlarged X-shaped chromo- 

 somes that will be discussed later. A pollen grain with its primary 

 nucleus in mitosis with 6 rod-shaped chromosomes present is shown in 

 PI. Ill, Fig. 36, the number being half of the normal number of 12 

 which is found in the somatic cells of the plant. That the number of 

 chromosomes at this stage may sometimes be greater than 6 is shown in 

 IM. Ill, Fig. 37. Thus certain normal conditions are to be noted along 

 with numerous abnormalities at all stages in the processes of sporogenesis. 



A development that produces four pollen grains in a tetrad group from 

 one pollen mother cell is. of course, to be considered normal in respect to 

 the number of spores formed. A functional pollen grain is one that will 

 act in fertilization and a fully normal spore is. we may consider, one 

 in which each nucleus possesses (i chromosomes (PI. Ill, Fig. 36). It 

 may, however, he conceived that pollen grains which carry more or less 

 than a. complement of ('. chromosomes may function in fertilization. The 

 hybrids obtained with the Europa clon are being studied for evidence 

 on this point. 



'I'm: Various Types of [rregularities 



We may further summarize the irregularities that occur during 

 sporogenesis in plants of the Europa clon as follows: (a) the increase in 

 the number of chromatin units as already described, (b) the irregular 

 distribution of the chromatin units, (c) the developmenl of anomalous 

 shapes in chromosomes, (d) the irregular and abnormal organization of 

 the daughter nuclei, (e) the omission of one of the reduction divisions, 

 (f) the Ion iijii hui of abnormal numbers of microspores with t he abort ion 

 of many of t hem ami (g) a failure in the proper divisions of the primary 

 or the generative cells in certain oi the pollen grains. 



