The comparative investigation of apices of vascular plants by experimental methods

Publisher Website
Google Scholar
Abstract
A considerable diversity of histological constitution is to be found in the apices of seed plants, and these again differ notably from those of eusporangiate and leptosporangiate ferns. Yet, in their growth and morphogenetic activity, the vegetative apices of all classes of vascular plants have much in common, i.e. they give rise to a vasculated axis with regularly disposed lateral members. Comparative investigations of apices by strictly anatomical methods have definite limits which are soon reached. In the present investigation the aim was to see if, when the same experimental treatments are applied to very differently constituted apices, closely comparable or divergent organographic developments ensue. When the shoot apex in selected eusporangiate and leptosporangiate ferns and in species ofPrimulawas isolated by vertical incisions from the lateral organs and tissues, with concomitant severing of the incipient vascular tissue or procambium, the apex continued to grow and gave rise to a short vasculated leafy shoot in which the normal anatomical pattern was soon reconstituted. Relevant data forDryopteris aristatahave appeared in earlier volumes of theseTransactions. Comparable data for eusporangiate and other leptosporangiate ferns are now described and illustrated. InPrimula, in and just above the region of the incisions, the vascular tissue of the new axial growth was in the form of an uninterrupted ‘cylinder’, conforming in outline with the triangular or rectangular contour of the isolated plug. The experimental materials have afforded clear evidence of a basipetal development of vascular tissue from pith cells, strands of the new vascular system eventually becoming conjoined with those of the parent shoot. In the leaf-bearing region of the new shoot the vascular cylinder was interrupted by leaf gaps as in the normal development inPrimulaand in ferns. A prevascular ring, with foliar gaps in the regions of leaf insertion, is present at the shoot apex inPrimula, this being comparable with the arrangements in ferns such asDryopteris.When all the very young leaf promordia were successively removed, the shoot was found to have an uninterrupted ring of vascular tissue, as in equivalent experiments withDryopterisand other ferns. The experimental data so far obtained thus show that when the same treatments are applied to very differently constituted apices, closely comparable results are obtained. The implications of this finding are discussed in relation to (i) the importance of the cellular constitution of apices in organogenesis, (ii) the the diversity of apical constitution in vascular plants at large, (iii) the apex as a self-determining region, (iv) the inception and subsequent development of the vascular system in different classes of plants, and (v) the relative contributions of axis and leaves to the vascular system.
Keywords