Graduate Thesis Or Dissertation
 

Root initiation and differentiation in Pseudotsuga menziesii (Mirbel) Franco

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  • Tissue differentiation of the primary root and its associated laterals is reported. Secretory elements are the first of the primary tissues to mature. They appear to be located between the precursory phloem and pericycle in the primary root, but are more closely associated with the pericycle in long lateral roots. The stele of the primary root is generally triarch, but may also be tetrarch. Lateral roots, however, are diarch except for an occasional triarch arrangement in rapidly elongating first order laterals. Some second and third order long laterals are mycorrhizal. Short roots are either degenerate or mycorrhizal. These short roots have no secondary growth or resin ducts. The degenerate short roots fail to develop a well organized meristem and abort soon after emergence from the parent root. This occurs prior to invasion by either saprophytic or mycorrhizal fungi so that fungus activity cannot be responsible for the development of a degenerate root. Mycorrhizal short roots are commonly racemose. The extent to which the fungus mantle covers the root tip appears to govern the development of the apical meristem and subsequent growth in length of the mycorrhizae. Root regeneration following pruning is reported in relation to the age of the seedling and the method of pruning. Adventitious laterals originate primarily from cambium at the end of a protoxylem pole. In two and three year old pruned roots, adventitious laterals originate in the cambial region of a vascular ray. Aside from their differences in origin, adventitious laterals develop in much the same manner as the laterals from which they are derived.
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