Mendel’s original experiments were concerned with factors which are expressed in that part o f the life cycle of the plant in which the nucleus is diploid, containing two of each type of chromosome. Pre­ dictions can be made from the chromosome theory as to the ratios to be expected for characters which are apparent in other phases of the life cycle, and also as to the course of inheritance in organisms with atypical nuclear cycles. These predictions form a valuable test o f the theory. We shall consider first the principles which apply to factors which are expressed in various phases o f the life cycles of plants, and then the results o f some different chromosomes cycles. The special case o f an abnormal chromosome cycle which is found in polyploids, i.e. in organisms which possess three, four, or more o f each kind o f chromo­ some instead of the usual two of each kind, is o f particular importance and is considered in the last section of this chapter.