An ultrastructural study of the phloem of some minute seedlings and of some primitive woody angiosperms

Abstract

The work described in this thesis is in the main an attempt to throw additional light on a very controversial aspect of sieve tube structure, namely the condition of the functioning sieve plate pores. The avoidance of surge artifact has been attempted by employing two main approaches, the second quite novel. In the first very minute seedlings have been fixed whole. The ultrastructure of their sieve tubes has been investigated and compared with that of maturer plants of the same species. In the second, seedlings, some minute, have been subjected to neutron irradiation to the point of flaccidity before being conventionally fixed for electron microscopy. The results of all these investigations have confirmed the usual picture of pores occluded with P-protein.

As a contribution to the understanding of the function of P-protein several species of primitive vessel-less angiosperms have also been investigated. These prove to have typical P-protein distributed as usual in the sieve tube lumens and pores. This material must therefore be understood in the context of transport in the woody plant since the herbaceous habit is recognised as secondary and P-protein only appears historically with the angiosperms.