The Comparative Anatomy of Extra-Chromosomal Types in Datura Stramonium

The Comparative Anatomy of Extra-Chromosomal Types in Datura Stramonium

The Comparative Anatomy of Extra-Chromosomal Types in Datura Stramonium

The Comparative Anatomy of Extra-Chromosomal Types in Datura Stramonium

Excerpt

Among the wide variety of traits in plants and animals which have been investigated by students of inheritance, those which are concerned with internal structure have been very largely neglected. This is but natural, since anatomical characters can ordinarily be observed only by means of a somewhat laborious technique and since the more evident traits of size, form, color and other aspects of the organism which are readily distinguishable without dissection have provided ample material for research on the problems of heredity. It is to be regretted, however, that so little is known as to the genetic basis of histological and anatomical characters, since they are obviously of the utmost biological significance. The sweeping conclusions of modern genetics have sometimes been questioned on the ground that these are based primarily upon a study of superficial rather than fundamental traits. Certainly for an understanding of the manner in which developmental processes are controlled by genetic factors, a study of internal structures is especially important, since every external difference, however simple it may appear, results from differences in an extraordinarily complex internal mechanism. The important interrelationships between genetics and morphogenesis can never be understood without a knowledge of the manner in which all the structures of the organism, both internal and external, are determined in inheritance.

Such work as has been done on the inheritance of anatomical characters in plants has concerned itself almost entirely with a comparison between hybrids and their parental types. Even before 1900 some important results had been reported. Thus Brandza (6) in 1890 found that hybrids between Æsculus rubicunda and Pavia flava resembled the former in the vascular structure of the petiole and the latter in the radial arrangement of the vessels in the stem and in the grouping of phloem cells. He also studied the anatomy of hybrids in Marrubium, Rosa, Medicago, Cytisus, Sorbus, Cornus and Cirsium. In some cases the hybrid resembled one parent more closely than it did the other. In other cases the hybrid seemed to be essentially intermediate in all respects. In still others, it resembled one or the other parent closely in some respects but was intermediate in others.

Essentially similar results were reported by Macfarlane (10) a few years later. He studied hybrid anatomy in a wide range of types and found that in general the offspring of a cross between two diverse parents was intermediate between them, though various exceptions to this rule were noted.

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